Lots of factors for weathering bricks, and painting them too...I try to paint different colours here and there for brickwork, just to break up the manotany of having one single colour. In rubble, once everything is glued down, and dry, I hit with a dark wash, usually burnt umber and black mix. some drybrushing, then depending on my scene, I add alot of brick dust, be it from MIG Pigments to Stardust Brick Dust..even mixing the two gets some neat effects.
Here's an example of what I mean...taken from my Linking Up dio....
Cheers,
Kelly
"There you go with those negative waves again!"