Excellent color on the rainbow. You even have a secondary one on the right side! There is a little too much noise in the dark clouds - Neat Image would take care of that very nicely.
Very nice. I wanted to chase a rainbow once ... to get it into position over something of interest. By the time I could get in the car and get moving it was too far gone. So you were either in the right spot at the right time, or you really worked for this one. Great job. Love the colors, love the composition (subject at bottom with rainbow running across the frame to lead you to it, negative space on the right). Good luck in the challenge!
Great photo. On my screen it looks like there is a lot of noise in the sky. You might want to try neat image or photoshop to get rid of the noise. Kevin