Don't forget about proportion though. If you get a thinner halter, it'll just highlight how wide the rest of her head is. If you get a halter that isn't flat and bulges out, it would highlight any bulges she potential has (if she does have any, I can't tell).
Interesting thought as well - if you cover the top half of the picture with your hand to cover the yellow/orange, your mare's coat looks lovely with your blue jeans. If you cover the blue, it's not quite as lovely. I think if you picked a color in the same tone as your blue jeans, that would work out really well. Not just blue, but the darkness - for example, I don't think your fellow competitor's blue shirt looks good with your mare.
Just my opinion, of course. I think both you and your mare are lovely. Agree also with the conditioning stuff that the others dicussed.