The two more common types of linen pocket squares for warm weather are white with a colored edge and colored with a white edge. I prefer the latter style, and the best of these in my opinion are the Alma line from Simonnot-Godard.
Simonnot-Godard has been weaving the highest quality cotton and linen on eighteenth century looms since, well, 1787. Beautiful stuff.
Find them at Alan Flusser, Paul Stuart and The Andover Shop in the United States, Turnbull & Asser in London and Arnys, Marc Guyot and Lanvin in Paris. Or, contact the company's showroom in Brussels by telephone at +32 2 742 01 04.