I'll stand corrected about "lilacs" vs. "rushes." Seems to me likely there's some chain of influence between the songs, if for no other reason than the striking coincidence of unusual phrase. I'd suspect common origin in some lost ancestor (the usual dodge for textual stemmatologists). About the derivation of the term "gringo," well, I have no personal investment in the point, but I also have enough experience with the making of dictionaries (especially etymological ones) to be as suspicious of the "griego" derivation as of the "green grow." The appearance of the thing in print has wonderful authenticating power. But without some documentary evidence, we have nothing to go on but plausibility, and I guess each of us makes our own determination on that one. A similar process yielded the term "goddams" for the habitually blasphemous English troops opposing Joan of Arc.