Should I use heat resistant paint when whitewashing a brick fireplace?
If you plan to "whitewash" it, I would highly recommend using whitewash (lime-based paint), not normal paint, though it's hard enough to track down these days that you may need to make your own. The face of the fireplace does not need to be particularly heat resistant (though the inside ends of the face bricks may get quite warm), but whitewash will be moreso than paint, and I think it works better with brick than most normal paints do. White painted brick looks ugly in short order.

As for black paint on the inside, good luck with that if you actually light a fire in there. That's asking a bit much, quite frankly. There are truly high-temperature refractory paints, but most of them are pretty-near white. Most (all?) black pigments burn off at high temperatures.