Going against theme here . . . how about a pattern? Something bold and easy to parse, like black and white stripes, say. Makes it bloody obvious it's no coincidence and allows for a great deal of variation and personalization, depending on freehand skill. But anything reasonably crisp and bold would do. Could even be something as simple as white with a large colored dot on the forehead. This is from a family funeral, which isn't exactly the correct spirit, but it conveys the idea. You can even tell relationship to the deceased from the color of the dots, if you know how to read that. (And if they're large enough for color to come across effectively.) A similar technique could serve to distinguish rank in our band. (Black dot=cap'n, red=first mate, green=master or some such.) Though truth be told, unless it's a full on pirate fleet the crew will be small enough that such devices wouldn't be necessary. Still, just a thought.
Also: armbands or sashes would be equally effective and wouldn't interfere with piratical headgear, but I don't have a picture of that.
I vote for stripes in two colors, but I like stripes.