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Amalgam fillings will tend to make teeth look grey for 2 reasons:
Firstly, the metal can be seen through the translucent tooth structure and secondly, if the junction between tooth and filling starts to leak, corrosion products can be absorbed into the tooth.
Either way, there are numerous ways of dealing with it and it is unlikely to require you to bring forward your dental appointment.

The remaining surrounding tooth is translucent so the colour of the filling will be transmitted across the tooth.  If the filling is large, the light transmission can be stopped making the tooth much greyer than it would normally have been. These affects can be reversed by having a white filling - have a look at my answer given in Inlays regarding replacing a silver filling. 

The dark filling will shine through the tooth making it appear grey. This is not necessary a clinical problem.  Potential issues can occur with very large amalgam fillings such as tooth fracture.  This is due to the tooth having little support or strength from the filling.
 
I would often place a porcelain onlay instead of a large filling as they strengthen the tooth and look great!

The colour is, as you say, from the large metal filling shining through the thinned tooth structure.  Although it may not look nice it may well be healthy enough.  It may well require a crown in future which would strenghthen it & make it look better.  Tooth coloured fillings may be appropriate but it depends how large the restoration is.