Why mexian called out green go home.?

It comes from a folk explanation for the origin of the Word "gringo," that during the US invasion of Mexico (in the 1840's) the Mexicans chanted "green go home." Actually though US uniforms at this point were blue (and the dollar was not green yet, either). Gringo in fact comes from "griego" which was what people in the Spanish empire called foreigners in general.