Lots of Mexicans call Americans gringos. One explanation for the term goes back to the Mexican War and the marching song the Americans sang incessantly that started "Green Grow the lilacs..."
Hmm, "Green Grow the Lilacs" was the name of the original play written by Lynn Riggs that became the musical, "Oklahoma!"
Seems like the song itself was really "On Top of Old Smokey," or the tune was the same.
I heard that same explanation from my husband's Roy Rogers CD.