The most basic and common color for chaps is black. I would go with that. And your boots need to be the same color as your chaps. And you can wear jeans under chaps, but it does not look as professional as wearing pants the same color as your chaps. Black jeans would be ok. I use an old pair of black dress pants under mine.
No sparkly belts, as they break up your lower and upper body too much. A belt buckle on the other hand is fine, and recomended if you tuck your shirt it, which most people do for a more fitted look.
Hats are usually black, white, or shades of brown. Black is the most common, and least expensive. And you want to have it at least somewhat shaped to your face, aka no flat brims!
Your hair needs to be in a neat bun, perferred with a hair net. It is distracting if it is down and flopping all over the place. Yes to the makeup.
As for shirts, a fitted plain button up is more than acceptable. If you go that way, a neck scarf can provide a great color pop. Mostly, you don't want something that breaks your body up. So a shirt that the bottom is the same color as your chaps and fades into another color is best.
If you need I can put some examples up of what I am talking about.