How is it acceptable in society to call people with red hair ginger, rojo, or wedo? But when it comes to other Ethnicities it’s considered racism?

Having red hair isn’t a race. So calling someone something derogatory due to hair color, isn’t racism. By definition.

To answer your implied question, insults or stereotypes about hair or other traits can be bad and rude and prejudiced, without being the same as racism. There’s a difference between schoolyard insults and systemic oppression enforced by state violence.