A Quick Look at Four Color Separation
These are the four plates used in the printing of the base image in the Two Toilets... Print; cyan, magenta, yellow, and black (cmyk).
Why CMY "K", and not cmy "B" for Black?
The black plateis refered to as "k" for keyline as opposed to "b" for black. Many believe this was to avoid confusing the b with the cyan plate (blue), but historically black was often referred to as the keyline plate, as it held the key information, while the other plates were merely for color, etc. In addition, until relatively recently, most presses could not print in register well enough, so the black plate would have a keyline border around the image. This would hide the out of register dots that appeared on the edge of the image - If you saw the dots, a layperson could see something was wrong, but would not notice the lack of sharpness in the actual image. Having a keyline would avoid all that.