I had a client, Sam who would paint big strokes in red and yellow. He would stop and look at the contrasting colours, cover his eyes and turn away. He then painted over the colours with white or blue until he had mixed colours he could tolerate. Then he would build up the painting with other colours he liked.
In doing this he was sort of playing with his visual sensitivity and by painting into the strokes he was altering or taking control over visual input. Art therapy is often beneficial as a means of engaging with the world and it's input. This includes learning how to interact and change things, which makes people feel more in control. From infancy children gradually learn how to interact with, coordinate and control things both internally (with toileting) and externally (holding liquids, eating, fingerprinting then using a brush).
For individuals on the autism spectrum external input is often frightening and painful. It is experienced as something imposed and inundating. Learning that you can moderate and change things helps encourage interaction with the world against the tendency to withdraw and block out by making repetitive movements or sounds.