Q: I noticed last week that one of my few tomatoes growing on my tomato plant in a pot was two-toned. I’ve never seen one of these before.
A: Very odd!
My guess is that incomplete pollination caused it.
Tomatoes expand and ripen in response to hormones produced by developing seed inside.
If one side of the tomato got good pollination and the other did not maybe you could get the result you see.