I just used colour burn to blend the 4 colours and the waves and that got rid of the 'grey' areas in between the 4 colours. Although, can't see much colour between red and blue...meh
Turned out really nice! Thanks!
EDIT: I would put this as my avatar (my current one is LAME, and I can't be bothered making a new one) but I don't think that's fair, I should be making my own pictures instead of stealing from the tutorials...