Whether I am painting a model constructed from wood, plastic or plaster castings, I always paint the mortar lines first and then when this is dry, pick out the individual bricks or stones with the appropriate colour(s). Yes, this is more time consuming, but results in a more pleasing finish and a nice variety of shades, etc. You don't say what scale you are modelling in. I am modelling in 4mm, which I would suggest is probably the smallest scale you would like to try this in for the sake of your eyesight and your sanity!