Modern paints are not poisonous, only the old lead based paints. There is a good reason for painting the entire box, inside and out. It equalizes the vapor movement on both sides of the wood, and the wood is much less likely to warp. Dipped hives will stay straight and true, when hives painted only on the outside are pulling nails at the corners from cupping of the boards.
And contrary to the popular notion, the bees don't mind it a bit. They are a bit less likely to gum it up with propolis, that's all.
I get my paint from a painter who saves his exterior latex to swap for honey. He drains the remnants of his one gallon cans into a five gallon pail, so the colors can be quite creative...