Rainbow has been used as a girls' name in the USA occasionally for about three generations now. There was a silent film called Rainbow in 1921 about a young woman named Rainbow who inherits a copper mine. It was considered a "hippy"-ish name during the 1960s and 1970s in the USA and was sometimes used then. Until a few months ago there was a newspaper columnist for the Omaha World-Herald in Omaha, Nebraska, named Rainbow Rowell. (She's now moved on to another job.)