I plan on installing cement board behind a wood burning open stove on an outside deck.
Do I need backing behing the cement board and can it be colored using high temp paint?

Cement board is pretty strong material but it is not designed to be exposed to the eliments. Sounds like you are trying to protect a flamible surface from the heat of the stove. I once did this by making a low wall of stacked bricks. It looked pretty good and protecetd the wood railings but you could not lean on it. A friend of mine later improved the wall by glueing the bricks with a caulk tube morter.