Photoshop's CMYK Proof: Why To Avoid At All Costs

When I was tossed into a graphics design job, I had very little understanding of CMYK. I understood RGB just fine, but upon learning that my projects aimed for print ads and the like had to be in CMYK, I went the ultimate newbie route: designing my ads in RGB and converting them to CMYK when I was finished, praying they would look okay. Sometimes they looked okay, sometimes they didn't. It was a complete crapshoot. I didn't want to simply design in CMYK because I got really washed-out gradients and I liked some of the filters that could only be used in RGB mode (something I use rarely if ever now, but when I was first starting out filters just seemed too cool to give up).

After getting frustrated with the drastic changes some of my designs took when converting them to CMYK, I did some googling to find out that you could design everything in RGB mode with a "CMYK Proof" on that would show what the RGB design would look like once converted to CMYK. I found multiple people not only listing it as an option, but actually suggesting that people use it for their designs. And it really did seem like it was the best of both worlds: access to all the RGB features while knowing what the final product would look like. Even for people who have little use for RGB filters, there didn't seem to be any downside to using the CMYK proof setting. I used this for ages with the only noticeable issue being that occasionally a final conversion does NOT look the same as the proof did. It only seemed to happen when I was using multiple style layers and layer blend options, I'm guessing it just came down to software errors, but simply flattening the image in RGB mode and then converting worked for me, so I thought everything was perfect.

Where I noticed an issue with the CMYK proof was when I was doing some test prints of a document I was making in InDesign. The document was a small little brochure of the products that the company I worked for was selling. Let's call the company MKD. The official logo for the company was an Illustrator vector CMYK file. The logo was simply the name of the company with CMYK values C = 100, M = 0, Y = 0, K = 0. In other words, completely cyan. For the brochure, I needed to make a little edit of the logo for one of the pages. It was going to be a one-time use, so I lazily made it in Photoshop instead of making a new vector in Illustrator. Like I normally do, I went ahead and used RGB mode with a CMYK proof. I dragged the original logo into Photoshop and hacked away at it, converted it into CMYK mode and then put it in my InDesign document. Everything looked good. When I printed the document to see how it would look, the original MKD logo looked amazingly bold - a nice, deep cyan. On the other hand, the edited one I made looked bad. The cyan was completely washed out and did not stand out at all.

I was confused, to say the least. Both images looked identical in color on my screen. The color I was seeing was this:

in both CMYK and RGB (when viewing them on my monitor). Look like a nice cyan, right? But for some reason it only looked good when the one made originally in CMYK was printed. To understand why, you have to understand a bit about how CMYK and RGB are different. If you've worked with CMYK before, you've probably noticed the fact that it's hard to get some of the boldest colors out of it. For example, if you're trying to use an RGB value of R = 0, G = 0, B = 255 (all blue), you'll get something like this in RGB:

On the other hand, if you convert that value to CMYK (or if you try to use RGB 0,0,255 in CMYK) you will get a color like this::

So, why the difference? CMYK uses subtractive color, while RGB uses additive color. When the red green and blue colors for each pixel on your computer monitor are off, the monitor is black - no color (roughly). Every time a color turns on, color is added. When color is added, the screen gets brighter and brighter and when all colors are on, the screen is white. It's not always easy to tell, but this animated GIF might help to show this. The gif goes from black to blue to cyan to white, RGB values 0, 0, 0 to 0, 0, 255 to 0, 255, 255, to 255, 255, 255. It becomes slightly brighter in each frame.

CMYK, on the other hand, is the opposite. It makes sense when you think about it. Monitors are black (no color) until colors are added, while paper is white (all color) until colors are removed. When my original CMYK logo was printed at 100, 0, 0, 0, it was printing cyan ink at its fullest. Since this is subtractive color, that means that to print cyan like that, the ink is removing all colors BUT cyan. Using the colors given in the CMYK format, to make a blue color, you must add magenta to cyan. So a decent representation of blue in CMYK is 100, 100, 0, 0. To make the best blue color in CMYK you might use less magenta than that, but 100, 100, 0, 0, should be pretty close, and it works for the purpose of this explanation. So consider the nicely printed cyan at 100, 0, 0, 0. This subtracted all colors BUT cyan from the paper it was printed on. Now, if you have to add more color (magenta) to get blue, you are adding MORE subtractive color, which is taking away from the color and boldness of the print-out. So when you are subtracting both cyan AND magenta, you're taking away a lot of color to get the best blue. The result is a color that looks blue, but loses a lot of the boldness and saturated look of the color. That is why the "CMYK" image above looks so much less bold than the "RGB" image, even though they are both trying to represent blue. When you add 100% yellow to that washed out blue, you ideally should get black. Because of the shortcomings of the subtractive use of ink, it ends up being more of a dull gray. Here is the RGB representation of CMYK 100, 100, 100, 0:

That's why the K (black) layer of CMYK exists, to add a rich darkness that cannot be achieved with just CMY (it also helps that black ink is typically quite a bit cheaper). If you add 100% black to that color to get CMYK = 100, 100, 100, 100, you will get the richest print black, and the RGB representation of 0, 0, 0 (no color).

Okay so now it's time to actually get down to why my printed logo looked bad when it was made in RGB, and why working in RGB for work that will eventually be CMYK is typically a bad idea. It's clear that using only CMYK colors to represent RGB colors would not look good (as shown by the lackluster CMYK blue that can be represented much better in RGB color). This is because CMYK has a different color gamut (range of colors) than RGB. It may initially seem like CMYK has a smaller color gamut than RGB, and one that can fit entirely inside of RGB's color gamut:

The reason it seems this way is that any of the CMYK colors you're seeing on your monitor are being represented by RGB values, so RGB seems to be able to show all CMYK colors, and it CAN show all the CMYK colors that you are familiar with seeing on your monitor. But that's exactly the issue. The colors that are unique to CMYK can never be represented accurately with a monitor. And unfortunately, the colors that cannot be represented by RGB are some of the best CMYK colors. So working in RGB for CMYK projects makes it so that you cannot use the strongest CMYK colors, the ones that stand out the most and look the boldest. A better representation of the color gamuts of CMYK vs RGB is something like this:

Keep in mind this isn't meant to be a quantitative graph, it's only meant to show that there ARE CMYK colors that cannot be represented by RGB. Among these colors would be CMYK: 100, 0, 0, 0; CMYK: 0, 100, 0, 0; CMYK: 0, 0, 100, 0; the three boldest colors CMYK can show. A simple way to test this is to create an RGB Photoshop document and a CMYK Photoshop document. Open the color picker, and enter in manually CMYK values of 100, 0, 0, 0. Then use the paint bucket tool to fill the image with that color for both documents. You will notice that the RGB representation (what you're seeing on your monitor) of both the RGB  and CMYK documents' 100% cyan color appears to be the same. Now, use the eye-dropper tool on that cyan color to see what its values are in both documents. In the RGB document, you will see RGB values of 0, 174, 240. In the CMYK document, you will see RGB values of 174, 239, 0. As you can see, the RGB representation in both documents is almost exactly the same, which is why they look the same on your monitor (side note: I'm not actually sure why they are ever-so-slightly different, I would have expected them to be exactly the same). If you do the same thing with the eye-dropper, but this time look at the CMYK values, you will notice a different story. The CMYK document has values of 100, 0, 0, 0 that we originally used: completely cyan. RGB on the other hand, has values of 69, 14, 0, 0. Now, as was discussed earlier, adding 14% magenta will subtract color from the printed document, making the cyan stand out less. But more importantly, the actual cyan color itself has dropped from 100% to 69%. That makes a HUGE difference on printed documents, and it (combined with the added magenta) explains why my MKD logo made in RGB looked so much worse than my logo made in CMYK.

You see, when working in RGB, the file stores the RGB values ONLY. It doesn't matter what you entered as a CMYK value or which added images are CMYK. The RGB pixel data is all that will be saved in an RGB document. The CMYK value that you get using the eye-dropper tool (or from converting from RGB->CMYK) is just a rough conversion from RGB to CMYK. What that means is that putting the rich cyan CMYK color of 100, 0, 0, 0 into an RGB document will only store the RGB representation of the color, which is something close to 0, 174, 240. This is NOT the same color, since the 100, 0, 0, 0 CMYK color is outside of RGB's color gamut. So you are losing the rich cyan color that only CMYK is able to represent when you put it into an RGB document. Then, when you convert it back to CMYK (or just print it in RGB), the color that is printed is the CMYK color that your RGB color maps to. Since RGB cannot map to the bolder CMYK colors, it maps to  the closest color it is able to: the ugly 69, 14, 0, 0. And THAT is the reason that working in RGB for CMYK documents is a terrible idea.

And that's the simple distinction, in CMYK your color is going to look beautiful, in RGB it will not. I know that might seem like a small difference for just one logo designed poorly, but the issues with that one logo apply to all CMYK image design. You could have entire images that get their colors cut down because you were working in RGB. It doesn't happen all the time because you have to use some of the rich cyans, magentas, and yellows to experience that issue. But here's the thing: knowing that those colors are the richest colors you can get out of CMYK, you SHOULD use them in your documents. You can create the absolute best CMYK images by using everything that an RGB image would NOT allow you to use. If you're like me, you probably hate how washed out colors are in CMYK compared to RGB. Well, if you use CMYK properly, you can create images that are just as bold as your RGB ones. The trick is simply knowing how to do so. I hope to touch on this in my next post, which will hopefully be up within the next few days. EDIT: Here is the first part of said post.

So my explanation for the RGB/CMYK issue is complete. If you're exhausted after reading all of that, I understand. I know it was a lengthy explanation. It might seem unnecessarily lengthy too, but I think hitting all the details is important. The main reason I made so many of the mistakes I did early on in my graphics design career is because people DID NOT go into lengthy detail. I simply heard a few people online say "hey, use CMYK proof" and I thought it was great. If I had known all the details I know now, I would've avoided it like the plague. My hope is that in going into detail like I do, you will be able to make your own decisions about how you want to approach your image design, and hopefully can come to your own conclusions about how you want to design, without just trusting uninformed advice from strangers. That being said, I have to admit that I usually do really enjoy discussing the nitty-gritty details of image design, processing, etc., and I want to thank anyone who took the time to read the full post. I put a decent amount of work into it, and it took me awhile to write, so if you gained some knowledge (or even if you hated it), I'd love to hear from you.. If you have any other comments, concerns, complaints, or questions, I'd love to hear them as well. Thanks!