Actually, whilst uniforms weren't issued until the mid 19th C, that doesn't necessarily suggest a total absence of uniformity. Certainly by the mid 18thC warships would have slops chests from which clothing could be purchased and that contained bolts of cloth that might be made up by taylors amongst the crew. That would impose a degree of uniformity in its own way simply as an exercise of limited choice. It was also fashionable and a quite regular practice for ships captains to outfit the crew of the ship's gig in a uniform livery at their own expense. Not too much of a stretch then to have 'uniformity' within a narrow range of colours on your sailor figures and a specific group dressed alike.