I am doing a series of little paintings using this technique and having lots of fun.
It is so exciting when it works.
The paint is used very thinly and is almost entirely transparent pigments: transparent oxide brown and red, raw umber, raw sienna. Then black (nose, pupils of eyes) and in the case of this lovely little chap, a touch of white for his muzzle and flash.
But the key thing is NOT to use white.
As soon as you introduce white the whole thing is inclined to fall apart. I had to wipe this lovely boy twice before he came together and the culprit each time was white.
Apart from his nose, all the other "white" in the painting - the light fur on his ear, for example - is achieved by wiping paint off with whatever comes to hand. My finger, the handle-end of the brush or a cotton bud (Q tip) and allowing the un-toned white of the board to shine through.