First off, here is a video of last years World Champion in the AQHA Amateur Horsemanship.
The attire for the HMS (horsemanship) is usually more darker colors (not to say that there aren't hot pink HMS outfits, lol). HMS is all about the rider, and the individual pattern work is first (and a lot of the time all you do) so you don't NEED bright colors, lots of sparkle or things like fringe to get the judges eye. Showmanship generally follows these trends too since it is also judged on the person. But in showmanship you would wear jackets or tunics instead of a fitted shirt that you tuck in.
Western Pleasure on the other hand, your in the arena with up to 25 or so people all trying to get the judge to look and remember them. So the Pleasure is where you will find all kinds of fun things! For the most part, anything goes in the western pleasure. The same is for the Western Riding and Trail even though they are pattern classes, they are judged on the horse not the rider.
Here are some examples of a few types of HMS outfits: http://www.roughterrainfarm.com/imag...egion6_000.jpg http://aqha.com/Showing/World-Show/C...rld.ashx?w=350 http://aqha.com/Showing/Select-World...ion.ashx?w=350 https://fbcdn-sphotos-a.akamaihd.net..._7225714_n.jpg
Me and my friend in our HMS attire. I'm on the sorrel! Lol
But the biggest thing when finding a HMS outfit is to think smooth lines. You don't want to break your body up. http://images1.americanlisted.com/nl...i_11207424.jpg
This I do not like at all. See how the white hat with the black chaps with no large spots of white in the shirt just looks odd? Also how the blue shirt kinds chops her body in half as opposed to the previous shirts which either fade into the design or are higher up?
Hope that helps. If you need more, just ask, lol. I love doing this!