Color Modes and Bit Depths
When creating digital artwork, one of the earliest choices is deciding on the color mode that will be used. While the decision is not irreversible, it’s beneficial to understand the differences between the two primary modes: RGB and CMYK.
Every time you take a picture with your phone or look at images through your television, you are utilizing the RGB color model. Red, green, and blue light combine in varying degrees to trick your eye into seeing a whole spectrum of color. Combining red and green light will create yellow; combining red and blue light will create magenta. All three in equal saturation will give you gray or white depending on the amount used and of course, black is a complete absence of light. Due to light being combined together to fill out the entire range of color, it is known as an Additive Model. It’s the mode used in graphics that are intended to be primarily viewed digitally.
Have you every wondered why graphic programs break RGB ranges in increments from 0 to 255 instead of percentages? This is due to the inherent way data is computed. You may have noticed options for “bit depth” for when selecting color mode (8 bits/channel, 16 bits/channel, 32 bits/channel). A “bit” in computer terminology–binary language–has only two opposing states: on (one) or off (zero). Stacking 8 bits in a row gives you 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 possible variations per channel (Red/Green/Blue). Not coincidentally, 2 to the 8th power gives a total of 256 possible states for each channel (or more precisely, 255 degrees of light plus 0 which equals no light).
That means higher bit depth yields a color gamut with more variations, right? Yes… technically. But it real world applications, the difference between using a bit depth higher than 8 bits per channel usually only means larger file sizes and more memory intensive work environments with results that are often identical to the human eye. Having 256 potential states each for red, green, and blue produces close to 16.8 million colors once those channels are combined. Creating degrees of difference between all those colors is impractical and almost always unnecessary.
While it makes sense to create digitally viewed graphics in RGB mode, printing companies will often ask for files in CMYK. This is a process color system using Cyan, Magenta, Yellow and blacK inks patterned together in different percentages to trick your eye into seeing a whole spectrum of color. Once light hits the ink, it bounces away, losing part of its brightness and displaying the color perceived by our eye. For this reason, it is known as a Subtractive Model. The values associated with each ink is a direct correlation of what will be printed, which can mean more predictable results if printed pieces are intended to be the final use of the digital files.
You may have noticed that CMY and RGB have a special relationship on the color wheel. Directly between any two colors from either model is one of the colors from the second model and the direct opposite of the third color from the first model. For example, cyan and magenta will make blue, the opposite of yellow. Green and blue will make cyan, the opposite of red. Since having RGB light values at full strength produces white and having no light produces black, it seems logical that the opposite should be true of CMY. Indeed, no ink produces white (or the color of the media) and printing all inks at 100% produces a dark color (black in theory but sometimes a very dark brown in practice). Black was added to the model to make printing cost effective and efficient as using only one ink instead of three to get dark blacks and neutrals grays reduces both ink used and drying time.
So digital files intended for print should always converted to CMYK? Not necessarily. These days, software RIP’s (raster image processors) do a great job of converting RGB files for direct CMYK printing. And since files are often intended for print and the web, it can be convenient to create files in RGB mode which have the added benefits of larger color gamuts and smaller file sizes. Again, the downside is color that’s not consistent with expectations. One example is printing a file in RGB mode that has solid blue (Red = 0, Green = 0, Blue = 255). Instead of being the blue you may expect by viewing it on screen, it may print much warmer, a color some may call as much purple as blue. Because RGB has a larger color gamut, it must be clipped or altered to print and since vendors all use different combinations of software, printers, substrates, profiles, and rendering intents, results can vary greatly from print house to print house.
The most important factors when deciding in which mode to create your graphic masterpiece is intended use, comfort with each mode and which benefits are most important to you. RGB has a larger color gamut, smaller file sizes and generally more pleasing on-screen appearance while CMYK tends to more consistent between printers with printed color books providing references for expectations. The best news is that either can be converted and manipulated if color doesn’t look quite like you expect.
For a great printable reference about color theory, including terminology and information about color schemes, click here.