What is a traditional colour for a garter? Should I wear a blue garter?

In short
There is no tradition for garter colour. Blue is a popular choice because it covers the 'something blue' of the old rhyme, and blue has long stood for purity and faithfulness. Pick whatever colour you like.
There is no particular tradition for the colour of the garter, but it is a great way to incorporate a 'something blue' from the wedding saying ‘Something old, something new, something borrowed, and something blue’.
This saying, which dates from Victorian England, was what a bride needed for good luck. It is also believed that the colour blue is considered to represent purity, faithfulness and fidelity.
The collection Wedding Garters See the collection