YAY I've cracked it - I can get it to change colour everytime.
Its easy - just don't fling the ball around too much and keep it going straight up rather than off at angles and you should be able to click it every time.
Fairly easy. Just press ALT to open up the menu bar, that releases the pointer from the string then click quickly double click on the ball. Cheating a bit I know
I got it to change 13 times in a row, was a little annoyed after the 3rd one when it went back to red but after that i've seen all the colours of the rainbow