I use the 'black prime, drybrush' method of painting brick structures, if the walls don't already come in a brick color.
Start off by spraying all the walls with black primer paint. Cheapest stuff you can find at a big-box. After it dries, use a large brush and stroke on partially dry white paint onto the black in random swatches. When you're done, you should have areas that are all black, areas that are all white, areas that are partially each in various percentages combo.
Then, drybrush your brick color onto the walls. What this does is three things: 1) on the black, you will get a darker colored brick. On the white, you will get a lighter color brick. On the combos, it'll be a mixture. This is basically a 'one step' weathering technique, because you get three shades in one shot.
After this dries, use a small brush (like an 00 or 1/0) and cover random individual bricks in brick color. This will highlight.
You may or may not want to do a wash to simulate the grout, but it's sometimes not necessary because the white in Step 2 may already do that.
You get the best results if the grout lines are deep. I do my best work on stone buildings.