Here is my process for making brinks with white mortar.
I start by basecoating the bricks black. Don't worry about white spots as that is the color our mortar is going to be.
Second I put in a deep coat of white between the bricks. Don't worry about stating in the lines yet.
Now I start with Terra cotta. I then use terra cotta as the base for lighter and darker shades of bricks. Try to stay out of the white motar. It isn't too hard as the bricks are raised.
You will likely get some color in the motar area. This is alright because it makes this next step easy. Go back to the white and cover the terra cotta spots in the mortar, but this time use your thumb and smear small amounts of white over the bricks. Do this in small amounts ove the whole brickwork.
I also do this same thumb stippling with black.
