Martinique is the island that Bertha's mother and Christophine are both from. It's referenced as the "Paris of the West Indies" owing to the fact that it's a part of the French West Indies and not a former British colony (Rhys 72). This geocode goes specifically to the town of St Pierre which is where Bertha got the dress that sparks the comparison to Martinique and Paris between her and Rochester. 

By including a reference to a French colony, it seems as if Rhys is drawing on the distinctions between the French and the British and the English prejudices against the French that manifest in the hypotext, Jane Eyre. This is far less subtle in Jane Eyre, especially to a contemporary audience, with references to Adela’s mother Celine as a French opera-dancer. In both texts, being associated with the French is conflated with being Other. Celine is continually dismissed and disparaged throughout Jane Eyre and both Bertha’s mother and Christophine are seen as dangerous and Other in Wide Sargasso Sea.

Rhys, Jean. Wide Sargasso Sea. W.W. Norton & Company, 2016