I did not make any precise plan as to how I want my rainbow to be. I knew I wanted it to be out of the cup or rather, coming into the cup. I took some colours from my watercolour palette and start making strokes of colour :) Simple!
When I have watercolouring on my projects, I like to start light. It is easier to add on than to take off :) This helps me with a lot of watercolouring disasters.
This card can be achieve by just having a one layer. But I was being silly and I couldn't get my rainbowed centered so I had to cut the piece and stick it to a card base :) A tip for you so that you don't have to start over after all the work.