Kind of like calling a redhead "blue", calling a redhead a "ginga" is ironic. Cause it's ghetto, and we're usually not. In a high school variety show, I was picked to ironically pretend I was a big hip hop/rap/hood rat person because I was the palest/most innocent looking/whitest person available. My pale, skinny, redheaded brother was almost named Mario, and I'm so glad that one didn't go over. It would have been equally ironic.
Anywhozizzle, the "ginga" part is funny because of the irony but the "ginja" -- see, I'm replacin the "g" with a "j" -- has substance. You see, redheads are borne out of a genetic mutation on the MC1R protein. So...we're mutants. Just like the Teenage Mutant Ninja Turtles! Ginger + Ninja = GINJA! Boo-yah.