Hi - if you search the Digital Tradition by putting rainbow sign, you'll find "Rainbow Sign" and two other songs, Sowing on the Mountain (click) and Sinner You'd Better Get Ready (click). Today is an appropriate day to ask for that song, since the "Noah and the rainbow sign" story from Genesis is one of the readings for today in the Common Lectionary, used by many of the Christian churches.
Seems to me there must be one or two black spirituals with the same line, but I didn't find them.
You can search the Digital Tradition from the blue box on the top of this and many other Mudcat pages.
-Joe Offer-