Yes. With time. Slowly the white plastic will turn yellow. Even if you protect it for six months, it'll turn yellow.
Transparent phone cases turn yellow because transparent phone cases contain sulfides, which turn yellow when oxidized. It is inevitable that the transparent plastic phone case turns yellow, when we use the mobile phone in daily life, it will be exposed to the ultraviolet rays of the sun and lights, and the plastic phone case turns yellow after absorbing the ultraviolet rays. However, the yellowing will not affect the overall quality of the phone case, and the hardness and feel will not change.