Weathering the viaduct means I need some brown. Chris Nevard uses a lot of emulsion paint when he works and while I've dabbled with it in the past, I've not really used it in anger on anything significant.
However, with a large area to cover, it's time to dig out the excellent Wilco "Nutmeg Spice" I use for groundworks (I'm on at least a pot a month I reckon, it would probably pay to buy the big tub) which should really be called "Dog poo brown" but isn't to avoid upsetting customers delicate sensibilities.
Thinned with water, I've washed it over the plastic bricks. Adherence is a bit patchy, but that only adds to the effect. This is only the first stage anyway. Basically, I slop it around and mop up the stuff that runs to the bottom of the wall with a piece of kitchen towel so we don't get a brown tide mark.
Proper modellers will use an airbrush and proper paint, but I didn't fancy taking the viaduct down to the garage for this, and anyway, on a project like this that isn't heading for a step-by-step piece in a magazine, I can experiment and have a bit of fun.