Magic Keyboard 2 key turns orangey/brown while cleaning with microfiber cloth

I have an iMac with a Magic Keyboard 2. I was cleaning the keys with Apple's microfiber cloth and some lens cleaner. However, one dirty key turned and orangey/brown and no matter how much I clean it, it doesn't turn white. I tried toothpaste, it helped a little bit, but its still that horrid colour. Anyone got any advice? This is really starting to bug me a lot.

Many Thanks,

MBA08.