A lot of the soft cases are made with materials that are reactive and will yellow from just the oils on your hands. Many things can cause the yellowing. No amount of washing will remove the yellow.
I am no fan of these soft cases as they also attract grit that can get trapped between your phone and case and cause scratching. A semi hard case will last longer and not be susceptible to chemicals that will change its color.
When you pay only a few bucks for your case, you can't expect too much.