Any recommendations on what to clean this with?

I just got this cheap at a flea market, more as an experiment on attempting to clean it up. I’ve got a couple versions of this mask and another rarer one I have has discoloration also. I figured this’d be a mask worth experimenting on. I only really need the filter to complete another kit for one of these masks in far better condition. Is there any way possible to bring the color back a bit?