Grey horses have a black skin and a mix of black and white hairs. They can vary in colour from black to almost white. 'White' horses are never refered to as white unless they are a true albino.
How the black and white hairs are distributed gives the variations of colouring e.g. a dapple grey has circular patches of darker hair; flea bitten has tufts of dark hair. Rose greys have the addition of chesnut hairs in their coats. All greys tend to get lighter as they grow older.
A rare colour, to be a true black the horse should be black all over, with black points.
Next - more colours.