The orange itself has been bugging me for a while too, as to me it's not really synonymous of a positive upvote. So sometimes I question if I've actually voted or not and have to unvote and revote to verify because I'm not sure if orange is the default or not.
I'd have thought green would be better.
Orange works on reddit because the upvote and downvote button are always visible and only the actioned arrow gets a colour when clicked, so you always know the state.