|Subtractive Color Spaces|
Most color spaces are additive or otherwise positively oriented; they treat black as a sort of default color when the relevant components are at 0, and white only when the relevant components are at their maximum. Some however are subtractive, such that white is the default, and as you increase the influence from each of the relevant components the resulting color gets darker, culminating in black when the relevant components are at their maximum. This corresponds to physical media such as inks and paint, where the role of the pigment is to absorb light, not produce it.
Make It Colorful supports two such color spaces, CMY and CMYK.
The CMY color space is essentially the exact inverse of the RGB color space. In RGB, the set of red, green, and blue component values (1, 0.25, 0) creates a reddish orange color, while to get the same color in CMY, you would use (0, 0.75, 1) for the cyan, magenta, and yellow components. Conversion between RGB and CMY is thus as simple as subracting each component from 1.
Likewise, whereas RGB is black when all components are 0 and white when they are all 1, it is the inverse with CMY where all components need to be 1 to produce black, and 0 results in white.
The CMYK color space is very similar, except that it adds an extra component, the key, which basically operates as a pure black component. A CMYK color with a key of 0.25 is like starting with a CMY color with each of the three colors already set to 0.25. It is essentially a light gray base that darkens any additional coloration that is added.
Ideally, at least one of the first three channels of a CMYK color is set to 0. If not, then that representation of the color is not canonical, because there's some darkness contributed by those three channels that could be moved to the key channel instead. This parallels the origin of the CMYK color space, color printing, where it was more economical to maximize the use of cheap black ink and be as conservative as possible with colored inks.