I have a bay Dartmoor and we use bright red at home, navy at training sessions and white when competing
Orange on a bay/chestnut.
Blues, black, white...any colors in my opinion. Lol
If your horse is a light bay try dark, rich colours like maroon with a neutral colour matching their mane or socks like black or white or a navy with gold piping. If it is a dark bay definaitely light pastel colours like powder blue with a dark blue piping or something soft like a baby pink, else go for bright colours like turquoise or green. It pops in colour on a dark bay.
My horse is a very chestnut bay and he looks gorgeous in purple
I personally would use dark red or something similar
Dark greens are my all time faves on chestnuts😍😍😍
My chestnut mare looks stunning in white!
Tourqoise I find looks quite nice if you're looking for a bright color.
And if you want to go for more classy colors maybe try maroon (depending on the bay/chestnut) or hunter green and navy blue.