Well, I don't water marble so I cheated and stamped and it's not a proper rainbow in the pretty pastel sense of a rainbow or a 7 coloured spectrum but it is lots of colours!
I started with a smoosh of 4 colours, Barry M Flamingo, Kiko 385 (blue), HJ Manicure Red Wine and A England Carnation Lily Lily Rose.
Then I stamped using Dixie Plates DP02 and Moonflower Turquesa. Then I added a second layer of stamping, same image, deliberately offset with Moonflower Violeta.
I think to be honest they were a lot more successful than I thought they were going to be and I wore them for 2 days. I really liked the combination of colours and I am quite pleased I didn't try to go for a proper rainbow.