Make sure you have the coins, and click one of the colors.
It'll highlight a color, you click the color (example Green).
Then it'll select another color to continue the pattern, (Green, Purple)
You then click Green, Purple.
Then it'll select another color to continue the pattern, (Green, Purple, Red)
You then click Green, Purple, Red.
If that doesn't work, what happened exactly that caused it to get stuck?