For passport name, I would normally try to take something from each parent’s name if possible (regardless of horse’s colour)
(Bad) example:
Sire: Clover Hill
Dam: Black Magic
Foal: Clover Magic, Magic Hill, Black Clover etc
For stable names, if a boy, I find that something nice and stuffy like Percy, Monty, Hugo or Oscar normally fits a grey/dapple ? For a girl, maybe something like Lila, India, Lydia?
Obviously also depends on the breed - ponies often get away with more non-human stable names (Snowy, Star etc), but bigger horses definitely suit human names. If it’s a foreign breed, it’s fun to name them a (stereotypical) name from that country, (Sven the Fjord, Heinrich the Hanoverian, Aila the Arab, Niamh the ISH etc)
Obviously, the ideal is if you can shorten some part of the passport name to a palatable stable name.