I don't use halfpad. I use full sheepskin pads. The half pads don't make much sense to me since the sheepskin doesn't come into direct contact withe the horse. It seems to defeat the purpose of using a non-slip moisture absorbing fabric like sheepkin. For colors, I use black for schooling. I have a nice clean white one secured in a plastic bag for shows.
I use a halfpad when my horse is first getting back into shape after a winter off - because he's not fit so the saddle doesn't fit him perfectly like it usually does, and I figure it can offer him some relief, so I use it. I'll also use it if I'm jumping, or if he's acting a little sore from a hard ride.
I use a black Steffen Peters Luxury Dressage pad which basically has the sheepskin 1/2 pad built in. I don't show so black was the way to go for schooling (hard to find in black though!) It stays put and provides all the advantages of a 1/2 pad without creating too much bulk.
I do miss using pretty colored pads for fun, but this gets the job done and cost a small fortune :)