Güero is a Mexican term for blonde
I'm also white in Argentina, Chile, Spain and probably every Spanish speaking country. Whiteness is not only defined by your skin tone in SA, facial features count a shitload when determining whether someone is white or not.
A lot of race mixing (Visigoths, Moors, etc) has happened in Spain throughout its history, that's why you can see milk white, olive, light brown and dark brown folks in the same family, which is very common in families of European ancestry in South America as well.