CMYK Issues

The basics of the difference have already been described. What hasn't been said entirely is that there is not a 1-to-1 correlation between every color in RGB and in CMYK. When you have a color that doesn't translate 100% the printing software will make a best guess. If it does that too often your final image can look color-shifted or muddy.

While your home inkjet is CMYK an offset printing press can use very different technology. Printing on an offset press can look different from your home computer and even from other printing presses. Printing presses are, in part, mechanical, and so they have to be calibrated to adjust the color to specific tolerances. If you are working with a publishing house that has in-house graphic designers (like a magazine with its own printing press) then the original photos may be color adjusted so that they print out well on the printing press. Even with one home inkjet I owned, I noted that if I adjusted the bright to +10% and the contrast by +5% then the image would come out closer to what I want. It was sort of a standard color correction for my specific printer. While offset presses are pretty complicated, the operator may have to make certain parallel custom adjustments to get his press to function as desired.

If you go with a printer who doesn't bother with color adjustments he may just print the file you sent him "as is" and you get what you get. Since CMYK can be printed by layering four different inks on top of each other, I believe that it's possible to get precise color adjustments if the image is "color separated" or provided to the printer as four separate image plates. You could, for example, then tweak the cyan plate a little to color adjust it.

With professional offset runs of any significant size they'll usually offer you a printed proof, to allow you to reject the print run, modify things on your end, or ask for modifications on the printer's end. This can be problematic if you use a print shop that proofs on a different press or printing system than the final press. This happened to us. I had an image that was designed in RGB and printed in CMYK on my home printer. Looked great. The proof print sent to me was done on a digital press. It was almost a perfect color match. The final print run was done on a completely different press and my blues color-shifted into the purple range. We were on a deadline so we were stuck with the result.

Newer versions of Adobe Photoshop allow you to layout in CMYK mode. This is not a "what you see is what you get" mode, since your screen is still in RGB, but there's less color shift from press to press when you design totally in CMYK mode, because you won't have to worry about different color translation software platforms shifting the colors differently on different presses.

The best way to guarantee color is to get a CMYK swatch book. It'll come on a specific substrate. It's worth noting that the appearance of a color really can change depending on the type of substrate it's printed on. It can look more vibrant, duller, deeper, flatter, etc. So, ideally you want to see color swatches printed on the type of substrate you'll be using for your final product. Then, you pick a set of common colors that you are going to use in your design. You go into your page layout software (that can design in CMYK and not RGB) and setup those swatches. Then when your printer prints those colors on a matching substrate it will look very similar to the original swatches if all goes well.

You can get an idea of how your image might print by going into your image editing software and saving the image as a CMYK image and then re-opening it. Your screen is still RGB so you won't see the true differences, but you may get some idea about the directions of the color shift. Try also printing the RGB image on your inkjet and then converting the image the CMYK and printing it on your inkjet. There you'll see the difference between your image editing software's RGB-to-CMYK conversion versus your operating system and printing software's RGB-to-CMYK conversion. These won't give you exact parallels with what you'll expect from offset, but it may give you an idea of the range of possible results.

If you can, design using the swatch system I described in a page layout or image editing software capable of CMYK layout. Short of getting a printed proof from your printer and discussing things with him, that's gonna give you the most reliable results.

Lee

i've had that- blues to purples, yellows to sickened greenish tint, etc. upping the picture resolution to 600 or 1200 and using brighter settings/lighter colors helps, but if anyone's got better tips, i'd be happy to hear them too.

Two of us have now mentioned the importance of color chip sets or color sample books. Again, as I noted, you really need to see the color on the appropriate substrate to see what you are going to get in the end. For instance the same color can look quite different on an uncoated stock as compared to a coated stock, and can look different again once a press coat is applied to the finished product.

I will add one additional point of note. In printing there is a measurement called a Delta E (the difference between two colors). If you care about the uniformity of a print job (say matching a print job to a proof or producing an expansion matched to an original product) then ideally you should research the topic of Delta E measurements and include a low, but reasonable one, in your contract with the printer. Lower end print houses may have never heard of it or may be incapable of measuring it. The limit of the difference you are willing to accept and the accuracy with which a print house can reliably hit a specific color exactly is called the Delta E tolerance. Leave that out of your contract and you can encounter some problems. I have found substantial Delta E shift within a single print run. Consider just a standard laser printer or self-inking hand stamp. If you go on too long the later impressions will be much lighter than the first one as ink/toner runs low. If your print house doesn't monitor the print run or has any problems with their printing equipment then you can get Delta E differences that are noticeable in a print run. Usually not on the same unique printed press sheet, but it can be substantial from different copies of the press sheet or on different press sheets. Games with expansions like CCGs can be harder to print due the desire for matching backs. I often buy games from smaller companies in the industry and can tell the difference between an expansion card and an original set card if I look.