Use the gradiant tool in Photoshop.
Here. Just read this link cause I'm lazy.
http://www.maagraphics.com/tutorials/rainbow-effect-in-photoshop.htm
^^^ This, although you'll have to meticulously outline whatever it is you want to taste the rainbow.
Or just using the magic wand tool and create a new layer with it's own opacity and fill levels
I just wanted to maximize the amount of time fly would have to stare at a giant cock.
that pic is fucking hilarious.
ive been reading that site for like 15 min...im amazed....and inspired...