Mostly playing with after image, here's one where if you stare at the black crosshair in the middle you'll start seeing green dots left behind.

I suppose it is probably worth mentioning that while violet and indigo both get faked up with red and blue and no green too, on your screen, they do have actual wavelengths.

Also you don't exactly have red, green, blue receptors, you have S, M, and L receptors and those loosely correspond to blue, green, and red but the L receptor, as I recall (this isn't my area so grain of salt) isn't a single 'hill' like the M and S receptor with one single peak but one big hill with a second much smaller hill that peaks out in violet and at that point exceeds the M receptor in sensitivity, though not the S - found a graph. So when violet photons are going in they are being absorbed by those L (red) receptors more than the M (green) and one imagine's the brain just interprets any spot that is sending more signal from L than M while S is being stimulated as 'purple', it doesn't care that no violet photons are striking anymore than it cares that a red and blue striped disc sent spinning and appearing purple is never emitting overlapping red and blue light, just that and individual spot has emitted both in a shorter duration than it samples. L and S are both waving their flags but M isn't, hence purple, because your brain doesn't know what a violet photon is or even a red or blue one, it just knows how loudly L, M, and S are shouting relative to each other and doesn't care if L is shouting because it saw a violet photon or a red photon, since L doesn't actually know which it just detected. Grain of salt though, not my area.