What colour should I use to paint brick walls?

If you were painting a red brick wall, to get a realistic effect, I would, after the wall had been primed with a suitable primer, paint each brick with a terracotta paint followed by a dry brushing with an orange colour and finally a light drybrush with white. Don't forget that bricks are not a uniform colour, so for best result you should use several close shades of terracotta colours.

If you want to make your bricks look old or hand made, add terracotta paint to primed brick surface, ensure full coverage of the brick then add a blob of terracotta paint and add a sprinkling of the dust from a broken terracotta flowerpot.

Take a shard of flowerpot and grind in to a fine power, with a pair of, break the pot in to smaller pieces until it is just dust and small fragments left, do this over a plastic tub and all the dust and fine pieces of terracotta can be retrieved and sprinkled. Don't forget those safety glasses when breaking shards of pot into dust. 

I would recommend having the dust ready to go before adding the blob of paint.

As you add the blob of paint, sprinkle the dust into the paint, cover the whole brick area with a generous covering of dust, then when paint is dry, tip excess dust back into plastic tub for the next project using bricks.

I use the dust from flowerpots, different colour house bricks, concrete blocks. In short I use anything that may give more realism.

If you were painting grey house bricks, I would say it is the same as above, paint your primed brickwork a dark grey when dry add a light dry brushing of a lighter grey followed by a dry brushing of white.

Brick dust method is obviously optional.