I use two paints for my whites (oils only!), titanium white and raw umber. This gives a nice earthy weathered tone to the shadows. I would be careful with the Paynes, it does give a nice shadow tone, but is a VERY powerful pigment and tends to take over. Most paramount though, it tends towards "blue" quite strongly. Probably better for a dapple (tend to be more to the grayish side), than a plain white horse. Anthony knows what he's talkin' about! He can certainly help. Good luck, take your time and maybe experiment on the side on a scap paper plate or a spare horse part to get just what you're after.
Jay H.
OKC