How to genurate rainbow colors on OpenGl?

I am drawing a jet/plume in 3D and I want to change the jet color according to the concentration. I want use rainbow color palette, but I cannot find it in my book.

I have problem to generate rainbow colors in 3D OpenGl. Anyone who know the combination about RGB to produce rainbow, please tell me and I am very appreciate it.

Thanks a lot!

F. Chen