Glows are for all three sets of armor, and can be used as many times as you want. However, there is no way to get a glow for only your helmet, and you can not get one as a gift. If you only got a glow for a single piece of armor, and it came from an engram, that is from last year's event, and will not work on this year's armor. The glows for the current armor are only available to buy with silver or bright dust, and you get the entire set of one color glows for each piece or armor in one package. You can not just buy single pieces.