I actually stamped the rainbows as well as the backgrounds with an acrylic block. The rainbow was colored with Zig Clean Clear Brushes and spritzed with water. This one is basically the normal shades of the rainbow; the second one (next one posted), I turned the colors 90 degrees - just for fun!
Both rainbows are sprayed with glitter spray for sparkle and both are placed upon an acrylic block stamped using Distress Oxide Ink.