I believe he paints the entire building creme (or white), and drybrushes it with various tones of red brick.
He'll have to verify that though! 
Rob is correct. I first paint the whole frontage in the mortar colour.
Once that has completely dried I drybrush the brick colour on, working from a dark to lighter shade.
This must be done at 45' to the brick courses so that the mortar isnt overpainted.

Build the colour up gradually as, if you use too much paint on the brush it will go everywhere rather than just on the raised brick detail.
I then paint random individual bricks in a couple of different shades, usually by adding dark brown or dark blue to the base brick colour.
The difference that painting a few bricks in other colours is significant, it makes the brickwork look far less uniform & hence realistic.
FYI the latest frontages are now on the OWS website:
http://www.outpostwargameservices.co.uk/buildings.htmlRailways arches, add-ons & pavements.
I hope to have painted versions done by the end of the weekend.