Commercial products like Robert's brick mortar come in a variety of colors. They are applied and then wiped off so that the color remains only in the joints. Art hobby acrylics, such as Ceramcoat by Delta, are available at craft stores. Off-white, sand, light earth and gray colors are all appropriate for mortar. Coloring the mortar makes a big difference even if you choose to keep the original color of the plastic model and not paint the walls.
Scotch tape works very well for most masking tasks.
Very small brushes may be used to hand paint these details. More expensive kits have the doors, windows and window sills as separate pieces, making them easier to paint and then attach to the structure after weathering. Add signs representing those that are painted or papered on the structure. Dry-transfer signs, like those from Woodland Scenics, are usually easier to apply than decals. Purchasing and using the burnishing tool for dry-transfers makes them easier to apply as well.