There's probably not a rule against it, but remember horses in general and especially hunter is a sport steeped in tradition. I would not enter any ring, much less an AMHR or any rated show ring with a bright purple halter with glitter on it. Sorry, but there is a reason leather is used so much. It just looks so much classier. It is really more respectful to the judge and your fellow exhibitors to follow traditions and use a leather halter. The bright purple is a bit much.
Now, that being said I love me some glitter hatters! I'm a hoarder to be honest. I keep one (ok, more than 1....) on hand and crisp and fresh and brand new looking for each horse in my trunk. I use it as a going out halter. That's what they're tied with at the show, wear under bridles at poker runs for the big mare etc. That way I can still show it off and dress them up with it! Where did you get it and does it come in pony size?! I need one in electric blue or hot pink for my filly!