If you cross your eyes so that the two + overlap, you
may be able to see the colour called "yellow-blue".
For each row, stare at the left image for 20-60 seconds, then suddenly change your focus to the center image. These specific types of colour are known as chimera colours.
All of these colours are known as "impossible colours", as they cannot be percieced by our eyes under normal lighting conditions, or in some cases, at all (yellow-blue and red-green are "seen" due to the visual cortex of the brain being confused).
See here:
https://en.m.wikipedia.org/wiki/Impossible_color (pun not intended, but appreciated)