All Urethanes like Thermoplastic Urethane, or Polyurethane turn yellow after UV-Exposure or oxidation. There's nothing you can do about it, except adding UV-Protectors during the manufacturing process, or simply adding color to it. That's why most kitchen sponges are dyed yellow or some other color, You'll hardly ever find white sponges. By adding UV protectors some properties like transparancy can get lost, thats why your case gets yellow, because there are no protectors in it.
It's the same thing with grey/white ABS Plastic, which ages very bad.
I'd never buy a clear TPU case