I would call your dentist and ask why he chose silver over white. There are a few reasons why he may have chosen silver, your bite...if you have a strong or "off" bite the silver is still stronger in some cases. In order to do a composite(white) filling the area has to stay dry (meaning saliva and blood can not come in contact till the filling is set) if the filling was far back or along the gum line it may have been imposable to keep the area dry enough. The last reason i know of for doing silver is money, your insurance (if you have it)may not pay for a white filling in a back tooth. Usually you can pay the difference but your dentist may have just put in what was paid for. There is a big price difference, What matters most is the fact that you are not happy with the work so it is perfectly acceptable for you to call and discuss it with your dentist. I hope this helps you....Good Luck!
Our dental student friend should know that the newer composite resins are plenty strong for pre or 1st molars and usually fine in all molars unless one of the things i said above come into play. This months Dental journal had a poll about this exact issue (amalgam vs. composite) and over 70% of dental offices do not use amalgam at all any more.