With the rubber it shouldnt really matter how much you use, just put a little bit and mix with water.. Use a brush to apply and scrub it into the rubber.. Do both shoes at once. Dude I am baffled at your concords. The only thing I can think of is that its a chemical stain based on either the foam under the tongue which is yellow.. Or something on your denim. My two cents, Id oxi clean them as well. Why not, you have little to lose and alot to gain. Your other option is to grab some paint from angelus and touch them up.. Since they are still new and clean, even if the colors dont match they should be fine after a few wears. With rubber and midsoles, my goto method is using a rubber gum eraser.. This method on really is good for white midsoles. I strictly use it for as such. It looks like this. (you can use a suede eraser, however it just doesnt work as well as the rubber/gum eraser does) Process is simple. Just test on the bottom or a small part of the shoes first.. reason being is that it can remove finishing or sometimes even paint if you rub too hard.. So I only would really recommend them on white midsoles.. Works amazing.