Personal opinion: what color outfit?
 

I'm looking for a new show shirt and need to decide on a color - I already have black chaps so preferably something that goes with those. This is my horse:

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I'm assuming western because you said chaps? Well personally I LOVE turquoise or light blue with chestnuts!

I agree with the turquoise or light blue. Deep purple looks great in a bright chestnut, too.
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Mint green, turquoise, baby blue, lavender, pale yellow
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My horse is about the same color as yours, you can do just about any color except red, washes the horse's color out.

If you're talking English, blue/white. If you're talking Western, have fun and go for fun & bright! I agree with waresbear though, no red! Be weary of pinks & oranges too. Some shades of those don't do a chestnut justice! I'd suggest a royal blue/purple!

everything goes good with bright neon lime green !

Love royal blue on chestnut coloured horse the best. Blue is also my fav colour. Purple like a royal looking one would be my second choice.

Hunter green!