There are many different color spaces out there aside from the ones we discussed in lecture today, and one interesting one is the CMYK Model (CMYK refers to the four inks used in some color printing: cyan, magenta, yellow and key). This model is used primarily in color printing. You can read more about it as this link: https://en.wikipedia.org/wiki/CMYK_color_model.
Color printing revolves entirely around ink, and since ink absorbs various colors of light, the CMYK model is subtractive. White light minus red gives cyan, white light minus green gives magenta and white light minus blue gives yellow. This is how the three primary colors of the CMYK model are formed. As opposed to an additive color model such as RGB, in the CMYK model white is the natural color of paper, while black is the full combination of colored inks (instead of the absence of light in the RGB model).
A technique called halftoning is also often employed with colors on the CMYK spectrum. Halftoning involves printing tiny dots of each primary color that are small enough that the human eyes perceives a block of solid color. If we could only print the three primary colors of CMYK in block there would only be 7 possible colors, however with halftoning a full continuous spectrum of colors can be printed.