why is my melanochromis auratus loosing its color "turning white on the bottom?"?

Question

its black with a blue stripe and yellow on its tail and fins

Answer 1

Occasionally, less dominant fish will have their colors fade when in the presence of the more dominant ones, but I would suggest checking your water levels and doing a water change just in case.

Answer 2

http://www.fishbase.org/Diseases/diseasesList.cfm?ID=2343&StockCode=2537

it's either a parasite or a bacterial infection.
The above link details the identification of the disease.
The below links offer info regarding your fish and the practical fishkeeping offers potential cures in the treatment search.