(WARNING: ACTUAL SCIENCE!)

I was reading through my latest edition of Science News lately, and I found out something quite interesting. You see, the brain receives color in two channels: R/G and B/Y. All colors seen are made by the brain mixing blue or yellow with red or green. However, no color can ever appear bluish AND yellowish, or reddish AND greenish, because the different responses of the color channels are in opposition. (It's like trying to flex your bicep and tricep at the same time: the opposing impulses cancel eachother out.)

However, recent research has shown that under special conditions, the opposing-color theory is incorrect. When two opposing colors of equal luminance are placed side-by-side, and the image is stable on the viewer's retinas, the border between blue and yellow will blur, and you'll perceive a solid field of Blello (or Reen, if you're using reddish green).

Sadly, Urple cannot be produced with this method (as pink and purple are not opposing colors) but it's kind of neat, don't you think?