For this puzzle I've made, guess what is the missing color. You can't use a computer to find the solution. But once you know you have it, you are allowed to use a computer, mostly to select a RGB color. I insist on the
no-computers tag: it is way too easy if you are able to search on web :)
Hint 1 — 8th of July
Most probably you already saw this set of colors. Moreover, if you saw it, I would bet you saw it first in your childhood.