What causes a colorful C-shaped mark inside an old Vivitar 28-85mm lens?

Hard to tell what it is without examining the lens in person. Could be something benign, like lens coatings, or something impossible to fix, like balsam separation. Based on the age, type, and manufacturer of the lens, I suspect it could be oil on a lens element. Supporting evidence would include a "greasy" appearance and "sticky" aperture. The problem can be fixed with a CLA (clean, lube, adjust).

If the lens works fine as-is (probably not), I'd consider leaving it alone if the affected element is deep within the lens. It's an old zoom with clear damage that may not be worth the cost to fix. If you try to clean it yourself, you risk being unable to reassemble it because old zoom lenses can be difficult to reassemble. Disassembly of lenses made by Kino Precision is complicated by parts that are unnecessarily glued together.

Here's a picture of an oily residue behind the front element of a Kiron 28-210/4-5.6 Macro MC prior to cleaning: