I use Mikes method of diluted latex paint. It has always provided good results. I have used white, grey and beige colors for the mortar joints.
On DPM modular wall sections that have smaller and deeper grooves between the bricks, I will use a 2 to 1 water to paint ration and let it sit on the wall longer before I wipe it off. For Korber and Ameritown buildings, I usually use a 1 to 1 ratio.
Alan Graziano