LOL...I NEVER thought I could wear a hat...or wear "loden" green but I let the lady who makes hats talk me into it (her clients all look amazing). Ha, even when I opened the box for first peek at the hat my first thought was "OMG/UGH - how am I ever going to wear that!" But I learned how to dress to it, now LOVE it and hate taking it off at the end of the day. Just needed a little shove to think outside my box I guess
Helmets can look fab too...especially with a very simple, fitted solid colored blazer or blouse and lap robe and maybe a neck scarf for dab of colour. Wild Rags - type scarves (western ranch-style silk or poly neck scarves) are all the rage here right now, they come in a gazillion colours/patterns so it's easy to find something for everyone. And they're great for keeping arena dust, or mosquitos, out of your nose/mouth while you're schooling!
Lap robes don't necessarily have to match your cart seat but they should blend (..ie not clash!). I'd recommend using a skirt-type pattern if you're going to make one...I tried making a straight square one but the fit wasn't very flattering if you're at all curvy like me.
For ideas on what colours to look for, google "Carriage Driving Show Pictures" and surf around to see which turnouts you like and are similar to your cart and horse color. It's fun to see what works and what doesn't.
And best tip I found on here somewhere (can't remember from who)...once you think you've got it together, have a dress rehersal with horse & cart and get someone to take a pic. It's amazing how colors that look good together indoors can look totally different outside in natural light!