Look at the static disk, then gradually spin it so that it rotates very quickly. At a suitably high rotation speed, you will see a uniform light grey circle.
How it works
The image that the human eye perceives is not registered immediately. If the disk rotation speed is high, different colours from the wheel hit the eye so rapidly that it is unable to tell them apart. Consequently, all the colours that reach the eye within a tenth of a second is interpreted as one signal.
Combining the basic colours to get the desired ones e.g. in an RGB model is how the television and computer screens work.