Are you doing breed type shows or hunter/jumper? I can't help you much with the breed show fashions other then most people match coat colors with their horse. In the H/J world you can't go wrong with navy and darker greys, with a subtle pattern like a plaid or stripe. I see some browns as well but I haven't seen as many this year. Black was faux pas for years and years but has just started coming in, but it's a black with a pattern!
I love the browns, greys, and greens for jacket colors. Blue seems way to plain, and black reminds me of dressage lol. But just so you know I show mostly intercounty, 4-h, and breed shows, so I am pretty much allowed to wear any color, as long as its nothing crazy like hot pink, lime green, hunter orange, ect.