One of these things is not like the others
Please go to the Question #16 post if you haven't had a chance work on this one yet, since the text below contains the solution.
Over on Twitter, several people chose black, with the quite-reasonable explanation that the others all come from emitted or reflected light, while black comes from the absence of light.
Perhaps a better choice for the list would have been white, yellow, cyan, magenta, given this alternate solution.
The intended solution relates to the color model used in four-color printing presses. Computer monitors and televisions use additive colors, where the combination of the component colors (Red, Green, and Blue -- RGB) add up to white. That works with light. But with ink or paint, adding all the component colors together produces black. To get richer blacks in print, a fourth color of ink is included -- Cyan, Magenta, Yellow, and blacK -- CMYK.