This is one of my all-time optical illusions favorite because if you connect the "two" colors, it seems like it was a third color:Here is another good one along similar lines to the first one :
The 'blue' and 'green' colours are in fact exactly the same.
that last one is better as an animated gif I once saw. The purple dots disappear in sequence, like something is going in a circle covering them up; stare at the + for long enough and the purple dots disappear and you see it is a green dot going around in a circle.