The Rainbow Bug is originated and tied by Leon Guthrie. You can see more of Leon's originals at The Flies of Leon Guthrie. See his notes about this fly below.
Leon's Notes: Well I was not planning in sending anymore patterns while trying to compile book, but I cannot resist sending you this one that works extremely well even for Salmon, especially the Orange version. Many anglers here are having great success on the river and fisheries with this one, and much talk is going on about it; simple yet so effective.