I agree you might also look into silk threads as someone mentioned. In addition, you might look into some of the newer lighter weight - smaller diameter, larger number- threads.
Superior makes Bottom Line in many colors and their light colors fade into the fabric very similar to monofilament. YLI, Madeira and other companies make similar threads too.
There is no concern about using polyester thread in cotton fabric - the old tlae was that it would cut through the cotton fibers. This is not true of current thread and current manufacturing process.
Superior is one of the thread companies that have a great website for all kinds of information, too.