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Color is the visual perceptual property corresponding in humans to the categories called red, yellow, blue, black, etc. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the viewer’s light receptors. Color depends on physical characteristics of an object, characteristics of the object’s environment such as lighting, and the characteristics of the perceiving eye and brain, making color a very personal perception that we all interpret differently.
A color model is an abstract mathematical model describing the way colors can be represented as sets of numbers, typically as three or four values or color components. RGB and CMYK are both color models.
The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue. RGB uses additive color mixing, because it describes what kind of light needs to be emitted to produce a given color. When all of the RGB colors are added together at their maximum value at once, white is the result, when they are all subtracted, black is the result. Light is added together to create form color out of darkness. RGB stores individual values for red, green and blue.
CMYK short for cyan, magenta, yellow, and key (black), often referred to as process color or four color, is a subtractive color model, representative of the inks used in printing. The CMYK model works by partially or entirely masking certain colors on the (typically) white background. Such a model is called subtractive because inks “subtract” brightness from white. In additive color models such as RGB, white is the “additive” combination of all primary colored lights, while black is the absence of light. In the CMYK color model, it is just the opposite — white is the natural color of the paper, while black results from a full combination of inks.
CMYK uses subtractive color mixing as in the printing process, because it describes what kind of inks need to be applied so the light reflected from the substrate and through the inks produces a given color. One starts with a white substrate, and uses ink to subtract color from white to create an image. CMYK stores ink values for cyan, magenta, yellow and black. There are many CMYK colorspaces for different sets of inks, substrates, and press characteristics.
The RGB color space is capable of producing many more colors than the CMYK color space. The range of colors that a color model can depict is called the color gamut (a certain complete subset of the visual color spectrum). Therein lies the danger of creating RGB colors on the computer that will later be reproduced with ink on a printing press. Many RGB colors simply cannot be reproduced on the press. For example RGB “Blue” when converted to CMYK, will end up purple. Why? The application (depending on the application and color management settings) will covert RGB blue into a combination of 99.6% Cyan and 95.7% Magenta, purple, and that’s what will print on the press. This is because the RGB color model has a larger gamut, than the CMYK color model. The range of colors that an RGB monitor can reproduce is quite large, the range of colors that CMYK printing can reproduce is much smaller. If you pinpoint RGB blue on the RGB gamut, then overlap the CMYK gamut, you will find that RGB blue is outside of CMYK’s printable color gamut. CMYK simply cannot reproduce that shade of blue. So, the application makes the closest possible match, in this case, a purple. Many colors convert reasonably well from RGB to CMYK, those colors that are within the overlapping gamuts of each color space. But there are many more colors that do not convert well, those colors are outside of the CMYK gamut.
It can sometimes be difficult to visualize the reason for color shift in color space conversion. The best way to see the color differences between the CMYK and RGB color spaces is to look at a color gamut comparison chart. The chart above plots the visible color spectrum as the large “horseshoe” area, and within that is a plot of the CMYK color space for “2200 Matt Paper,” along with multiple RGB color spaces. You can see that the RGB color gamuts are much larger than that of the CMYK space. Colors outside of the CMYK gamut will be affected by conversion from RGB to CMYK, possibly with undesirable resuts, to the nearest CMYK equivalent value. If a designer sends a file with RGB colors to our prepress department, those colors will at some point before printing, either in the native file, or in the RIP, be converted to a CMYK color space. We assume that the designer knew what the result would be, after all, it is not our place to second-guess designer’s intentions, but rather to make their files print‑ready.
Additionally, the CMYK gamut varies depending on the press and printing conditions, the types of inks and types of paper used. Different presses, different inks, and even different press‑operators combine to produce slightly different CMYK gamuts. Just as important for a gamut dependent on reflected light is the underlying paper and how it interacts with ink. The same inks printed on uncoated newsprint and on coated bright white paper will produce two very different gamuts, the reflective stock will have a larger gamut because it is capable of reflecting more light. And as the perceived strength of each primary color is determined by the size of its halftone dot, paper absorbency has an affect on the gamut as well.
And of course, the lighting conditions at the moment the viewer percieves the colors of the printed piece has an enormous effect on color, simply because color is all about light and the reflection and perception of light. Viewing colors in a dark room, compared to a bright room, under warm lighting or cool lighting, in natural sunlight, or the dim light of evening, all affects how we perceive the final printed color. Colors can be affected by ambient light, reflections of other objects nearby, and the colors that surround the printed piece being viewed. To see some examples of the effects on perceived color by surrounding nearby colors, have a look at the eChalk website, and take a look at their Color Perception illusions.