I’m new to this too & experimenting with my first scratch build using embossed plasticard. I first sprayed the sheet with Halford’s grey primer and then painted on a dark brown as I wanted to portray old bricks. The paint I used was Revell Matt 84. You can see how dark this first looks on the right. Once this had dried I painted over the entire area with a light grey emulsion from one of those test pots I had kicking around in the garage. This was immediately wiped off the surface of the bricks but remains in the mortar lines. It also has the effect of lightening the bricks themselves. I then use a range of fine-tipped water-based felt pens (red, orange, brown, black) to highlight individual bricks and provide some variety. These are immediately dabbed with kitchen roll to dull them down a bit. I’ve still got more work to do to finish this but thought I’d share it as I’m pleased with the effect so far.