This is how I painted the (red) brickwork on my Geezers buildings:
http://leadadventureforum.com/index.php?topic=7181.1051. I painted the whole thing with Sandstone (B&Q smooth masonry paint tester pot). This needs thinning slightly with a little water over the brickwork, to ensure that it gets down in between the bricks. This then forms the mortar. (you may not need to thin the paint on plastic)
2. Once this has dried completely (overnight reccomended) I then drybushed the brickwork with Etruscan Red* (Sandtex masonry paint sample pot). This needs to be done at 45 degrees to the bricks so that it doesnt go into the mortar course. Gradually build up the paint on the bricks, rather than trying to do it all in one go.
3. I then randomly painted individual odd bricks using the Etruscan Red with a bit of dark brown or blue added. this gives a randomness to the bricks which looks much more realistic. Go and look at some (older) brick buildings to see what I mean.
4. Finally a light drybrush with the Sandstone to bring all the colours together.
*I used the Etruscan Red rather than Brick Red as it is a less intense (& slightly more orange colour) which seemed to suit older faded brickwork.
The technique should be fine with resin or plastic buildings