I've concocted a creamy pesto sauce, which is wonderful over pasta, but I've run into one aspect of it I don't like and would like to know if anyone can offer some insight. The sauce begins with a basic white sauce, to which I add my own fresh pesto. The problem I've noticed is this: if I make the sauce a while ahead of serving it, and it sits in the pot, the top becomes very dark, and looks ugly and unappetizing. I have to be careful to stir the sauce frequently to avoid this. Any idea why this happens, and what I can do to prevent it?