In Feng Shui, black and blue belong to water, so if your five elements belong to water, I suggest you choose black and blue as the phone case. In geomantic omen, red and purple belong to fire, so if your five elements belong to fire, I suggest that you can choose red and purple as the color of mobile phone case.
In the theory of five elements, white and milky white belong to gold. Therefore, it is the best choice for people who like gold to use white and ivory mobile phone cases.