Please help me do a spectrum recolour in GIMP

[beany_bot]

Hi, I want to recolour this cat. But not 1 colour. I would like it to be a spectrum. so going from red to blue across the cat, (I'll remove the white backround) through the whole spectrum. (like the colours in the second picture. I've tried it a few ways by cutting and so on but can't get it right and loose resolution by cutting it a few times. There must be a simple way to do this?

 

[leezer3]

Very brief twiddle:
Decrease to 2 colors.
Resize the spectrum image to match the cat.
Paste the 2-color cat in, and convert the black to alpha.

Getting it better than that will be a massive pain in the neck though.

 

[beany_bot]

Thats amazing! thank you. Ill need to try it myself though as the actual cat image I am using is 6m wide... lol.