For those not seeing it, try staring at the dot area for 30+ seconds. Then immediately look at the dot area in the other image, not at anything else first.
You're not seeing an afterimage containing different colors? Only lasts a few moments. One has to look at the original image, staring at one spot without moving one's eyes, for a while, then look at the same spot in the other image quickly.
It's the same effect that happens if someone takes your picture with a flash and you see a little colored (usually blue) square floating in space for a short while. It's retinal retention.