The spinning disk below is a little seizure-inducing, but it's a lot of fun because not everyone sees the same colour when it spins. Some people see green, others see yellow, a few see red, while some see no colour at all.
The disk is called Benham's disk. It creates an illusion of colour when black and white patterns rapidly change (even when there are no colours at all in the image). We're imagining colour where there's no colour to see.
The phenomenon is not entirely understood. One possible reason people see colours may be that the colour receptors in the human eye respond at different rates to red, green, and blue. More specifically, the latencies of the center and the surrounding mechanisms differ for the different types of colour-specific ganglion cells.
I got yellow and a little bit of brown.