I think is real far fetched that the color of you car will change engine heat.
But, if your going with black, beware, that color makes imperfections stand out the most out of any color. And White is the best color to paint you car to cover up imperfections.
Can't tell you why but i knwo it'* true. Ask a painter.
I would seriously consider painting it white agian. It will cover up any little dings and it will match your jambs. (painting the jambs sometimes ends up costing as much as painting the car since it has to go in the paint booth twice)
Oh and what ever color you decide to paint it, take everything off your car you can. Like head/tail lights, trim, spolier(bring it w/ you of course) , rubber around doors ect. It makes the paint job 100 times better.
just my $.02