Why Colors Shift when converting from RGB to CMYK
You have all seen it; the dreaded color shift when you convert from RGB to CMYK. I didn’t realize until today that most people don’t know why it happens. It is simple- the RGB primaries can produce a much wider gamut of colors than the CMY primaries can.
This odd little graph on the left is 3-dimensional graph of an RGB and a CMYK profile in the CIELAB color space. CIELAB (more accurately CIE 1976 L*a*b*) is a mathematical color space that computers use to understand color. There are several spaces, but CIELAB is one of the most common in the graphic arts industry.
Take a look at the graph. The brightly colored solid area in the middle is the graphed color gamut of the SWOP color space. SWOP is the standard for magazine printing, and most common color space people use when they convert their images.
The outside area (in wireframe) is the graphed color gamut of the sRGB color space, which is the standard color space for the Windows operating system as well as many consumer brand digital cameras.
In looking at this graph, you can see that the sRGB space is much larger- it has a larger gamut. You can also see where the largest color shifts will occur- the green region of the sRGB space is much larger than the SWOP, so those neon greens in your design will not print as bright as they look on your monitor.
This difference in gamut is why color management is so important to the design and print world these days.