Query: How to emphasize rainbow?

I have a 4000 dpi scan of the image shown below, original is Ektachrome slide. This photo is from March '96 at Chichen Itza.

I have quite a number of similar images, so it will be real easy to clone out the people from the image.

Using GIMP, how would I go about emphasizing that rainbow? I want to keep the dark skies, of course, but would just like to bump up that rainbow a bit. I don't want the rainbow to go 'over the top', just make it a bit more dramatic.

And, yes, I'll crop out some of the foreground in the final image.

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I used the pen tool in PS (I don't have Gimp) and I traced the rainbow and stroked each color on it's own layer and gave then a blur. With a little work I think it can work well. Great shot!

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Thanks for replies - MrAl, that's very close to what I had in mind.