People will still refer to redheads as "ginger" but only some of the time

That's a fairly new thing. Growing up, I never heard anyone use "ginger" in that way. It was always "redhead." Or sometimes "strawberry blonde" for lighter shades.

Millennial checking in, ginger was used all the time when I was a kid

Only a ginger should call another person ginger.

https://www.youtube.com/watch?v=KVN_0qvuhhw

The redhead, ginger, color link comes from early season ginger, which has reddish pink tips around the leaves right above the fresh root.

In UK English ginger isn't exactly a color either. It's not like yellow or orange. It's a state of being.

I don't know why this is being downvoted. Does "ginger-colored" sound right to you? It doesn't quite work.

It's true. A person is ginger, but nobody ever says things are ginger coloured. Ginger isn't even really one colour - it's a spectrum of orange-containing hair colours.

If you would then please explain the "kick a ginger day" from SouthPark

South Park popularized it. It was known but not common before that where I lived. It also popularized harassing them in general at my school.

Were there many reds around when you were growing up? My family is full of redheads, and ginger was a well known term to me. I'm much older than South Park

No, only a couple that I knew of. My area was mostly Mexican, then white, then native, then everything else.