Look at the static disc. Then gradually make the disc spin, so that is rotates very quickly. At a high enough rotation speed, you will see a uniform bright grey circle.
How does it work
The image that the human eye perceives is not registered immediately. If the disc rotation speed is large, different colours from the wheel hit the eye so rapidly that it is unable to distinguish them. Consequently, all the colours that reach the eye within one tenth of a second are interpreted as one signal.
Obtaining desired colours by combining the basic colours, e.g. in RGB model, is the basis of how television and computer displays work.