I’ve recently purchased a very nice grey suit. It fits me perfectly, however it is only a two-piece.
I need to attend a wedding soon and would like to get a waistcoat. They didn’t sell waistcoats to go with my suit, so I’m not sure what to do.
Should I just get a dark grey waistcoat which doesn’t quite match due to shade variances, or should I buy one in a contrasting colour? If so, what colour?

A dark red, burgundy type guarantees success.

Don't try grey you will never get exactly the right shade. For a wedding any bright colour should be OK.

powder blue.

A dusky pink will look a treat

the wrong person in here!

If it's a dark grey suit, another vote here for a deep red/burgundy colour.