Different materials used in fillings are different colours.
You can have an amalgam filling which is almost silver in colour, darkening over time so that it is quite noticeable and generally used for only the molar teeth.
White composite fillings are growing in popularity due to their high aesthetic value. Being white they blend in to the natural colour of your teeth. Porcelain is another filling material that is a natural tooth colour.
Gold fillings are sometimes used in the same way as amalgam fillings, again, normally being used for the molars.
It is up to you how concerned you are about your fillings being noticed. Generally back teeth are less obvious if filled with amalgam or a dark colour of filling. The front teeth are normally filled with white composite material.