Just a thought. Many baitfish have colored tails/fins. Most Notably Threadfin Shad's 2nd most prominent(after the spot) small feature is a gold/yellow tail. 2nd most notable is the number of small minnows with tail spots.
Heads are typically drab by comparison. Based on this I would think tails are the thing to dye. I wonder why more Drop-shot worms don't have shoulder spots?
Tight lines,
Wade