A useful tip just picked up while reading Chris Nevard's website is to use ordinary DIY emulsion paint (B&Q 'pebble' is the colour Chris used on the crossing keepers cottage on Catcott Burtle), thinned down with water and a little washing up liquid, as a wash over the top of enamel-painted brickwork (and then gently wiped off the brick surfaces) to recreate the pointing. When I painted my goods yard office I used thinned down enamel paint over the brick colour and then wiped off with a thinners soaked rag, which caused a real headache in that it was very difficult to remove from the brick surfaces before it dried and without leaving small fibres from the rag stuck to the paintwork, leading to what I feel is a less than completely satisfactory appearance close-up - and it took a heck of a long time for such a small building! The obvious advantage of emulsion is of course that it can be removed relatively easily with water even when touch-dry (or completely dry if given a real good scrub!). Something to remember on the next building I do.
Another ideal use for emulsion is colouring exposed ground surfaces such as bare earth, around sidings, platform and road surfaces etc.