Shadowfax was one of the Mearas, and the greatest horse at the close of the Third Age. Ask anyone what colour he is, and the answer will almost certainly be "white". This image appears to have been created by Peter Jackson, but I will pose this question: is Shadowfax really whiter than the other horses in the film?
On this film screenshot, we see a Gondorian horse conveniently next to Shadowfax in Minas Tirith, and the comparative results are rather unexpected. The Gondorian horse is actually whiter than Shadowfax, who is usually considered the purest of all the horses of Middle-earth.
Interestingly, the name "Shadowfax" comes from Rohirric "Sceadu-faex", which means "shadowy grey coat." In the books, in the daylight his coat appeared silver, while at night it was a shadowy grey that made Shadowfax nearly invisible in the dark. So the conclusion is that Shadowfax miniatures can, unexpectedly, be painted silver - and there is also justification to use GW's "Shadow Grey" paint, creating a blue horse.
These make some interesting deviation from the standard colour scheme.
(Mind you, I wouldn't try it, though
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