18k gold turns red?

I've had 9ct., 14ct. and 18ct. Gold turn red, and yes that is just tarnish. You can buy pre-treated polish cloths for jewellery and polish and it will go back to its original colour.
 
While I haven't heard of anything turning red before, I know I can't wear anything less than 14 and preferably 18 because the acids in my skin turn jewellery black.

I learned this the first time when I turned a beautiful 9kt (?) garnet ring from Germany black, black, black.

From what I gather it's not the gold that's oxidizing, it's one of the metals they mix it with.
 
I got yg gold earrings off of ebay that were little orangey-reddish on the backside. I cleaned them and polished them and they went back to yellow. I'm careful to make sure I keep them clean now.