It's the least trendy discipline. In regards to what you are judged on. That being said, it definitely has its own style. So while you won't be docked points, you don't want to look like a barrel racer or WP and stick out.
Close contact saddles are the thing. Dark oil is starting to show up on a rare few new saddles... but most are med to light oil. I like dark and chestnut, so feel free to help shift the trend!
Saddle pads- match the shirt. Yucca Flats are the in blankets for top shows. At other levels any will do as long as its clean.
Chaps... a must have. Fringed. Usually sand or black, sometimes varying shades of tan or brown.
Hats- black, tan, or straw. Simple is fine. But lately Shorty's Caboy has been pushing some drop dead gorgeous designs ($$$$) for reiners and it's gotten picked up by some top female riders. I'll drool and stick with what I've got. Big thing is shape your hat! It doesn't have to be fancy but it better be shaped and clean.
Shirts- the gals have been putting on the glitz more lately. A classic long sleeve plaid or solid button up is never wrong. But you can turn up the volume a bit if you want. Just don't go all Studio 54 disco ball on it. Unlike WP you aren't competing in a ring with other riders, so all eyes are on you alone anyways. The over the top glitz isn't needed to help draw the eye.
Pants- jeans, or I see gals wearing tan or black to match their chaps.
Your boots- Square toed. Usually sand or black, sometimes varying shades of tan or brown. You can even get away with lacers.
Headstalls- one ear is the norm. Sometimes a two ear. Other than that it can be fancy or simple. Unless riding a younger horse in a snaffle, then it should have a browband/futurity knot. I don't see a lot of crystal conchos. It's more metal or rawhide accents.
It's a lot like nuts and bolts - if the rider's nuts, the horse bolts! ~Nicholas Evans