I've actually used a couple of different methods. First the buildings are given a coat of good quality flat matt emulsion paint from testers brought in the usually DIY outlets, the sandstone paint in the picture is used for the stonework but is a bit too coarse for mortar, For the factories I have used either Laura Ashley Powder Grey or Craig & Rose Tintern Stone, previously I have also used colours from the Farrow & Ball range. I have tried the cheaper emulsions from Crown, Dulux etc., but they seem to flake off plastic or resin surfaces during the second stage of the process.
For the Northlight style factory the brick colour was added using pencil crayons in a limited range of reds & browns. The ones I have are from the Faber Castell range, these are rubbed across the surface of the bricks holding the crayon at an angle, so the colour goes on the face of the bricks, but doesn't go into the mortar courses. Talcum powder is then 'pounced' onto the surface using an old large brush and then brushed / blown off. This has the effect of blending the colours and also matting the surface. For a more weathered finish, weathering powders could replace the talc. The 'muck' on the windows is a mix of talc & grey weathering powder.
Initially I used this process on the larger factory building, but I wasn't entirely happy with the result, so some of the bricks were then overpainted with thinned Humbrol 70 Brick Red which darkens them, but still allows some of the original colour to shown through to provide some variation. I then went over the brickwork with some more talc. The brick arches above the windows were coloured using a grey crayon and the stonework is the smooth finish Weathershield sandstone.
For a building in the centre of the back scene I am experimenting with painting the bricks with brick red paint and then rubbing the crayons over them to add variety and depth of colour, before using the talc.
Hope this helps, let me know if you need any more info,