I'm not a guru designer either but I think that the rare ones also stay in sight for less time than the uncommon ones, which in turn are visible for less time than the common ones. I spotted a pinkish star shell during my game; it was the last of the shells I needed. By the time I'd found a kid (a couple of seconds) it disappeared. I had to wait for at least 24 hours before I saw it again (and bagged it). Of course, it may have appeared and disappeared when I wasn't looking.