Hi Nick

Mike is of course right about the knobs being plastic.

I believe they are, more specifically, bakelite.

They are not supposed to be painted (if they are).

And I think they are supposed to be what's commonly called "rose tan" in color.

I would remove all the tape you can, and then start with a cloth soaked in alcohol.

Alcohol shouldn't hurt the bakelite, but it CAN be very hard on paint, so just be aware of that.

Hope that helps.