I have in the past used emulsion paint (from ‘tester’ pots) for mortar: paint basic brick colour (in enamel), pick out random bricks in darker and lighter colour, then brush the emulsion on and quickly wipe off (you may need to use a damp cloth) - this mortar layer has the effect of toning down the brickwork and blending in the differently coloured bricks. The tester pot I used was a ‘mushroom ‘ colour- you could adjust the colour by adding eg black before painting it on if you wanted darker coloured mortar on parts of a wall.
Sorry I can’t add photos as I’m using phone.