I do have a set of eskadrons with flexisoft which are great - I have dark green for stadium at horse trials. I also have a set of Veredus, they can be expensive - I snagged mine off of ebay
for like $65, but they are very nice boots. They have lycra inside instead of neoprene which is good for horses with sensitive skin. I have these in black for jumpers.
You can't use boots in hunters, the only thing you can have besides saddle/fittings/bridle is a standing martingale - and only for the over fences classes, not on the flat.
If you ride in an equitation class then you can use open front boots or black polos, the boots seen are mostly leather (same color as tack). In hunters everything should be pretty discrete and conservative. If you are going to ride in the jumper divisions then you can use whatever kind of tack and boots you want (unless it's illegal... you'll have to read the rule book for the association).