RGB vs. CMYK
The main reason I decided to write this book is because so many of my students have had the miserable experience of sending their files to the printer and getting back something that is radically different in color from what they were expecting. They know something is seriously wrong, but they have no idea what it is—and therefore cannot fix it.
This chapter discusses what is wrong, and how to begin fixing it. To start with, let us have a closer look at RGB and CMYK.
First, what does RGB mean? R is red, G is green, B is blue. You probably knew that already, but would you know how to get yellow using RGB? It is extremely difficult to think in terms of RGB, and yet that is the method your computer uses to display every single color. RGB is the format that scanners and digital cameras use, too. So, all the images you see on your computer start out in RGB format, regardless of what is done to them afterward. There is just no getting away from it.
We saw in the last chapter how the CMYK colors work, so surely that is the most important format to know about when sending files off for printing. If that is the case, do you really need to bother about RGB? The answer is yes, because unless you know what to avoid, and how to avoid it, RGB will forever be getting in your way, and you will never feel confident about whether you will obtain the printed colors you want.
It will not work merely to change everything into CMYK as soon as it is on screen—even though it is essential that everything you eventually send to the printer is in CMYK format (unless it is a multi-channel “extension” of CMYK that also contains spot-color information—see the “DCS” section of Chapter 7). So, while the change to CMYK does need to be made, it is that change, and the way in which it occurs, that usually causes the problems.
The first thing we need to understand is that RGB refers to different colors of light, whereas CMYK refers to different colors of pigment. We have all developed some level of understanding about the way pigments mix together from the time someone first gave us a paintbrush and a box of paints. If there is a color we need but do not have, we probably have a good idea of how we might create it using the “primary” colors. These are colors that cannot themselves be mixed, but from which we can mix a whole range of “secondary” colors. For instance, if we want green—a secondary color—we would mix together the primary colors blue and yellow. For purple, we would mix blue and red. For orange, yellow and red, and so on.
All the paints we have ever used share this characteristic, and CMYK is just a particularly limited range of paints: we get only four colors, and everything else we need has to be mixed from them.
RGB is, in a way, even more limited because there are only three colors in the palette. Yet these colors mix in such a different way that it is possible to obtain a much wider range of colors from them than from CMYK. Unfortunately, many graphic designers have no idea about what the result will be when they mix RGB colors, because nobody ever gave them a paintbox containing colors made of light.
As well as being fundamentally different color systems, there is another big problem with CMYK vs. RGB. RGB is almost the exact opposite of CMYK, but not quite, and that is the cause of many color-related problems for the graphic designer. By the end of this chapter I hope you will understand what I mean by “almost exact opposites.”
CMYK and RGB are different in other ways, too. Because they are made of light, RGB colors can be seen in the dark. CMYK colors, being made up of pigments, cannot.
CMYK is called a “subtractive” system because the C, M, and Y components in theory combine to absorb all light and produce black. Of course, in practice, they do not. RGB is called an “additive” system, because it does (almost) the opposite.
Can you trust your monitor?
If you have ever had the experience of having an image that looks a certain way on screen, but then prints to look radically different, your lack of control over the change from RGB to CMYK is extremely likely to be the reason. However, it may not be the only reason. If you decide it is a calibration issue, read Chapter 4 for grayscale, and Chapter 6 for color images.
When you convert an image from RGB to CMYK mode, your monitor has no choice but to display the new CMYK version using RGB colors. So, an additional problem is that, as far as final printed color is concerned, your monitor cannot be trusted.
Incidentally, there is a very small range of colors within the CMYK range that you cannot produce with RGB. Pure cyan is one of them. So is pure yellow. But do not worry. Strangely enough, even though these are two of the primary CMYK colors, it is very unlikely to be a problem. Problems are much more likely to be caused by being unable to create an RGB color using CMYK than the other way around.