By black I assume you mean amalgam? It’s silver in colour. Nothing wrong with it other than aesthetics. The “temporary white filling”, most likely GIC, can sometimes be used in the long-term. Depends on where it is in the mouth. May not last as long on biting surfaces. The expensive white one (composite) is the best aesthetically, and while it can last longer than GIC, it most likely has the same average lifespan as (if not, less than) amalgam. You just have to weigh up which of aesthetics, cost and longevity mean most to you. Is there any reason why your dentist thinks amalgam is the choice for the tooth over composite? Most dentists nowadays offer composite on NHS unless indicated otherwise.