I was prompted to try this monochrome study initially because I loved the little dog's face, but the rest of the reference photo was not useable and I thought it would be a means of making a nice painting out of a poor beginning.
There is a long section in Richard Schmid's book "Alla Prima" in which he describes his approach to monochrome studies. He prepares his board using a lead based primer. This is essential for his technique, which is that - basically - he uses dark paint only and wipes it back to the lead primer to the extent necessary to achieve the lights. He does not use white paint. This wiping technique only works on supports primed with lead paint.
Unfortunately, you can't buy lead-based paint in the UK without a permit from the Department of Bureaucratic Affairs and to get this permit you have to swear an oath on the heads of your children or the children of your community that you are going to use this paint ONLY for the purposes of restoring and conserving historical artefacts (as opposed to what, I wonder? Re-painting teething rings for babies and selling them on eBay?).
So, I have to use white paint. My two colours were Rembrandt Transparent Oxide Brown and Gamblin Warm White. I have found it essential to get the darks down first without any use of white whatsoever. Once the white gets on the painting, everything starts getting lighter and lighter like turning a dimmer switch in a dark room and once you have started turning that switch the dark is never so dark again.
The other complication is that brown is a transparent colour and white is opaque. So all in all, a fiddly little piece but I think he is a sweetie.