Red-green and yellow-blue are the so-called "forbidden colors." Composed of pairs of hues whose light frequencies automatically cancel each other out in the human eye, they're supposed to be impossible to see simultaneously.
The limitation results from the way we perceive color in the first place. Cells in the retina called "opponent neurons" fire when stimulated by incoming red light, and this flurry of activity tells the brain we're looking at something red. Those same opponent neurons are inhibited by green light, and the absence of activity tells the brain we're seeing green. Similarly, yellow light excites another set of opponent neurons, but blue light damps them. While most colors induce a mixture of effects in both sets of neurons, which our brains can decode to identify the component parts, red light exactly cancels the effect of green light (and yellow exactly cancels blue), so we can never perceive those colors coming from the same place.Life's Little Mysteries