There's a challenge!
I would start by painting the brickwork with an all over coat of a brick shade to match your location - Matt finish acrylic and lightly sprayed on if possible.
Pick out odd bricks at random in a variety of different shades - darker and lighter - check out a full size wall for comparison.
When fully dry, wash over with a very dilute mortar colour that will seep into the mortar joints, and wipe and dab the wash off the face of the bricks before it can dry. This in itself will add further texture to the brickwork.
Weathering powders can be used to add rain splash up the walls - sand colour lightly brushed up the wall from the base. Careful, powders can go a long way but It can be worked off the surface with a bristle paintbrush.
Green powder brushed down the walls to simulate moss from leaking gutters and down pipes.
White powder mixed with decalfix can simulate the lime stains from water leaks on brick and stonework and a little soot powder can be used where water runs down from sills and lintels.
But most of all, practice first.
Other methods are available.