The National Health Service will not usually allow white fillings on the biting surfaces of back teeth. However, this does not apply to the sides of back teeth or to any front teeth.
There is a slight chance that some people may be sensitive to the metals used in silver amalgam fillings. If this sensitivity is proven, it may be best to replace the amalgam fillings with another type. (In very exceptional cases these replacements may be available on the NHS, if a consultant decides that a patient is extremely sensitive to these materials and asks for them to be replaced.)