I did my chestnut mare in burgundy once, only because I had left my chestnut skein of yarn at another barn and needed to get a passport picture for her ASAP. I think it looked nice enough, but I never did show her with the coloured yarn. She was a bit of an “unconventional” hunter (i.e. she’d rather be a steeplechaser), so until we made the switch to the jumper ring, I did all I could to blend in rather than stand out!
I used to braid my little grey horse in a pretty mossy green. I thought it looked beautiful, and blended very nicely with his grey and white mane. Dad’s bay mare always gets navy to help when he takes his braids out, but I prefer to do my own bay mare in black.