depends on the shade of bay I guess and it's whole look but I like black or a deep royal blue or deep maroon even as it looks professional but also white can look good but I'd say if you have a white saddle blanket on a bay put white boots on too otherwise the back points on the legs overrule the white and make it seem tacky.
I love royal blue on bays :)
Blue, white, black, maybe red...
I think blue or maybe burgundy?
It honestly all depends on your preference! However, I like a dark red or hunter green on most bays.