You can either use colored cardstock or white and color your own strips with markers or Distress inks or alike.
Remember one circle gives you two rainbows or more if you cut it up smaller. I ahve to make 14 cards now....If you're very particular about the order of the colors, I would suggest to use white card and either diecut each ring by itself or do like I did and stick them all together and make one pass through your diecutting machine.
But if you're making loads and loads of cards, this is the best solution I think.
I stuck all the colors together on the back with some sticky tape, then added tape runner and placed it on my cardbase. Also used taperunner for the Stamping Bella set of unicorns and foamtape for the clouds (memory box puffy clouds).
The sentiment is from Simon Says Stamp (believe) and I used Versafine onyx black ink, Then added some generic enamel heart/dots to help your eye travel across the card. Upor down you will just follow those hearts...try it ;-)