Painting back wall

[pirate]

Well I've drilled my tank and started to collect my peices to do the plumbing for the sump, but now I've run into another question. What do you paint the back panel of the tank with? Inside of the tank or outside? What color or is it a preference? Pros and cons to the color choice. I think it will be either blue or black and painted on the outside.
Thanks for all your inputs on these matters