My normal method for colouring brickwork involves mortar colour paint and a selection of brown colour pencil crayons. Sadly, for some surfaces, this doesn't work as the brick mouldings are too sharp and mortar lines too thin. This is a common problem with laser-cut walls for a start.
Needing to paint a building made this way, I've had to experiment with motar colouring.
A - Bricks painted and cream colour acrylic paint washed over the surface. In theory it will sit in the mortar lines. Not for me it won't. Wiping the surface doesn't help, not does abrasion with very fine emery.
B - Moratr lines filled with Deluxe Materials Perfect Plastic Putty filler. This works very well but on plasticard, it fills part way up the rounded (caused by the embossing process) brick faces and the lines are too thick.
C - Ready mix DIY store all filler wiped over the surface first by finger and then by high-density foam to clean the faces. By far the best although it needs practise and doesn't work so well on less pronounced brick faces.
Interesting experiments. Before anyone suggests adding mortar colour paint into the courses where capillary action will pull it along the lines - not for me it won't. I wish life was simple!