I have sketchbook Pro on my ipad and find it very useful for color experimentation, as well as shape, and composition sketches.
When I get proficient at using this tool - look out color wheel!!
(I hope you are all having a good weekend. I was shopping with friends yesterday but after that awful shooting in Conn, some of the joy went out of the day. My heart goes out to those families. I can't even imagine their pain. )