Slowed down, with a pause between them: a red dot, then a blue dot, neither moving.
Tricks of motion
Color phi
A dot seems to move across and change color. When does it change?
Halfway, though there are only two dots, a red and a blue, flashing one after the other.
Why? Strangely, you see the color change before the second dot appears. Your brain makes up the movement after it sees the second dot, then shows you the whole story.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)