"Some where... over the rainbow"

deepdive

Sorry, I couldn't resist the silly title.
I tried to capture some pictures yesterday of a rainbow. I will probably not be lucky enough to have this happen again any time soon, but I was wondering if anyone had any suggestions to get the rainbow to really pop more. I used my 60D, 17-85mm Canon with ISO 100 and app priority mode and had ok results. I set the app to 5.0 and the shutter speed was around 300. Depended also how I composed the picture (sometimes was a little faster). I shot some pictures and turned the exposure down 1 and up 1 to see how the results would look. I also tried 1.5 both + and - exposure and 2 was too much. The underexposed 1 and 1.5 really made the color of the rainbow more visible, but the sky and foreground were dark. Being in the positive it started to get washed out a bit.
My thought is that HDR (which I have not played with) would help, but I was wondering if anyone had any other suggestions???