At the risk of stepping off the strictly traditional platform, I would suggest you consider wearing the same vest/waistcoat as the other ushers. If theirs are black, just wear the one that you got with your jacket, but if they are in grey, or buff, or whatever else the bride likes, may I suggest that you do the same thing? Believe me, your plain sporran will be fine. Whatever colo(u)r hose you chose, someone will want to know its significance, unless it is black. You have months to come up with a good story as to why the groom's brother traditionally wears red or yellow. Just make sure your knees are visible and not pasty.