You've highlighted the rainbow really well. It's just a pity it's so small. And I can't help feeling it would be so much prettier with those trees in healthy green.
Very well done! This is a great application for selective de-saturation. I truly feel there needs to be a reason to apply the technique.
My only nit-picks for improving this shot would be to give it just a little more detail in the shadows and maybe a little more sky to put the horizon at about 1/3.