the term gringo is not from "Green go" because they did not have green uniforms yet

Just to reiterate, the Gringo doesn't have a bad connotation in spanish, at least in Peru. But it's usually to describe a blonde caucasian, a black, chinese or dark haired american wouldn't be a gringo per se. Even peruvians who are white or light skin w/ blondish hair are called cariñosamente (lovingly) gringo.
Versao

Gringo in Peru is not pejorative. Not necessarily US citizens...Australians are gringos, as Germans or Irish...

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'Necesitamos amantes para nuestra alegria, prostitutas para satisfacer el cuerpo, y esposas para criar a los ninhos y cuidar la casa'...Demostenes

Gringo is a mixed word from a western movie.The actor in this movie was call like this because his blond hair.However I suposse is because plants in desert(western movie,cowboys) are such as straw,and similar to the hair of white people.

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"Si un amigo te invita a almorzar,falta si quieres;pero si te necesita,acude como la sangre a la herida que no espera que la llamen".