I signed up for the latest class over at Online Card Classes - Rainbow Maker. So many different ways to create rainbows or use rainbow colours on your cards.
Each class offers at least 3 video tutorials on the assorted techniques, I highly recommend them!
Sharing a few cards I've created so far:
Distress ink blending.
Ink blending with a stencil brush, I used distress inks.
Ink blending with Distress Oxides using 3 colours only.
I had my friend's kids over the other day and she gifted me with these flowers which have served as the perfect rainbow background for some of these cards: