You're on the right track already - don't forget to prime your plaster pieces well first or the plaster will suck up the paint. There are lots of previous posts on this topic. Also, check out this great site - Doc Dio's World of Dioramas at http://fp.docdio.f9.co.uk
/ - it has good articles on getting started. Remember to vary the colours you use on the bricks and paint in the mortar between the bricks to avoid the 'toy wall' effect.