Posted this in the Pixel umbrella, thought I'd post it here, for the sake of having all tips nearby.
Open a new layer under whatever your working on.
Pick 2 colours you want to blend.
Make a linear gradient, from one edge to the other.
Use the posterize effect, and choose how many colours you want to use.
And you're done! Now all you gotta do is pick the colours with the colour picker and use them. You can delete the gradient blend layer afterwards.
Some might call this cheating, I call it creative use of advanced tools.
EDIT: Example.