An all-white horse is just 1 entire white spot. He’s still a bay/black/chestnut/whatever, but you’d need DNA testing to know what he is, unless the parents make it obvious (ie if both are chestnut, then he’s chestnut, if both are homozygous for black and are black, then he’s black).
Given that the small number of W16 horses are all or mostly white, I’d expect the same from any offspring if they inherit that gene
Not all W genes behave that way though. Some produce normal white markings, like W20 which is thought to be, or boost, the “sabino” gene which isn’t, and responsible for a great many normal leg and face white. W5 (Puchilingui, TB line) likes to produce a lot of body roaning but also just very splotchy/jagged body markings. W10 is common in the QH, and can range from normal white to lots of wild body white.
So it all depends on which W you are talking about, in terms of what to expect. Some produce “normal” white markings, some are normal until homozygous (when they produce dramatic to all white). Some produce all white when present, like this W16 appears to, and like the W2 (White Beauty/Patchen Beauty line), and W14 which has produced the all-white line of Japanese racehorses starting with Shirayukihime