The color is spelled with one "l" and I do think that that was how it should have been spelled. The town got its name very early though, and a lot of American spellings hadn't been standardized yet. If you ever want to feel better about your spelling, get a hold of William Clark's writings in his journal for the Lewis & Clark expedition, which was the same time when the town got its name. Clark spells "mosquitoes" four different ways on two facing pages while trying to explain how bad they were in the heat of summer in central Missouri. He would use different spellings for other things in the same paragraph, sometimes the same sentence. It was shortly after that when Noah Webster's American Dictionary began to be viewed as the standardized spellings, but that was too late for Vermillion, SD.