Ok so I plan on going with:
Khaki color for the pants. Black coat. Does it have to button up high? I have seen a bunch of jackets with different levels of buttons. A white shirt that has the high collar, no tie? .. black field boots. And a black helmet. Does it have to have a strap? I have seen a bunch of HUS photos without them but the advice above states that I need one? I am doing QH and Buckskin shows.
Khaki is alright, as long as it's not too green, there are so many different colors of breaches out there. A trip to you're local tack shop will give you an idea. Black coat is fine, hunter coats usually have 3 buttons, but top button tends to be about at the base of your sternum. White shirt is fine, hopefully it comes with a choker, a piece of material that you wrap around the high collar. For a little bit extra you can add a small pin in the center of the choker, adds some "bling" lol. As for you're helmet, you should check your rule book. If the show that you plan to attend says that it is run by AQHA rules then this applies:
Helmets are mandatory for all youth under 18 in fence classes and when schooling over fences. The AQHA does not require they be ASTM/SEI approved. Helmets are optional in all other classes at AQHA approved shows.
Now that doesn't mean that you shouldn't wear a helmet... I always ride with one, but that's personal opinion. :) So, if you are over 18, you can wear a hunt cap, which looks like a helmet, but has no strap. A side note: Make sure you're hair is tucked up under you're cap. It used to be the fashion to wear your hair in a little net and bow the stuck out at the base of you're neck, and is still cute for little children, but most people have their hair completely tucked under their helmet. The easiest way to do this is with two hair nets and a hair scrunchie. Put one hair net on, making sure to tuck away all the little hairs in front, then gather the excess hair net and the rest of your hair into a pony tail. Secure with scrunchie. Flip you head down and take the second hair net. Start at the base of the head and pull the hair net towards your forehead. Twist the extra at the front and tuck it in. With your head still bent over carefully put on you're hunt cap. The tighter you make it the better it will stay.
Hope that was helpful, and not too much