Why do Mexicans call Americans “gringos”?

Some say that during the Mexican-American war at the end of the nine-
teenth century, locals heard the invaders singing “Green Grow the
Lilacs” and simply picked up “gringo” from “green grow.” Others say that
because the American uniforms were green, the expression came from a
rallying cry: “Green, go!” But, in fact, gringo is a Spanish word on its own
and is a slang insult for anyone who is fair-skinned and looks foreign.