"He has horses in different colors."
Translation:Er hat Pferde in verschiedenen Farben.
Yes. It isn't accusative because... well, it's not as easy to explain as I imagined. Let's say there is no metaphorical movement, the horses aren't going into those colours (for example changing into different colours), they are statically in those colours, if this makes sense to you.
When they changed into certain colours, potentially yes (but I can't think of a phrasing that would allow that). But for describing the colours they had at any given moment, dative would still be needed. I go out of and into my house every day, sometimes multiple times, but when I am inside I'm still “in mein
em Haus”. It doesn't matter if it was the result of a change, a state is a state is a state.
Von is used for the ideas of location and cause, not necessarily the same as the English meaning of 'of'. Check out this web page: https://yourdailygerman.com/von-verbs-locations-german/