Paul the first thing to do is Google images of Dappled Grey horses , you will be surprised how much variation there is from almost white to slate grey . If you'r happy using both acrylics and oils then I would suggest after you have primed it give it a coat of very very pale grey acrylic then using oils start adding very small dabs of slightly various darker different greys over the horse and blend them in but leaving different tones over the body and give this a day to dry back . The next stage is to add the dapples and shading , for the dapples I like to paint a series of dark grey or black rings were I want the dapples , mainly on the flanks, hind quaters , belly and neck then very gently roll a piece of fine foam over them or stipple with a wide brush (the foam that most kits are wrapped in is good) this breaks down the sharp edges . now go back to your lightest grey and add a spot to each of the dapples and gently stipple them out but not so that you lose the the darker outer ring . Black or dark grey legs look good on painted dappled horses all thought this is not always the case on real horses but it makes that part easier to paint . Just add shadows as normal .
chippy