The definitive way to keep pesto green?

Another possibility to consider -- some nuts can discolor foods. Check that out.

Another good thing to do is add a little baby spinach to the basil. It has a neutral flavor considering all the other dominant players in pesto, so it doesn't affect the final product much except for keeping it a lovely green color

Blanching the basil works well, and covering the pesto in olive oil is great, too. But another trick I've discovered (and I doubt I'm the first) is that, if you use other greens instead of basil (I especially love to do this with arugula), the pesto will stay a pretty green. Of course, it won't taste like traditional basil, but it's mighty good nonetheless.

Before you place it in the refrigerator, pour olive oil over it so that you have a layer of olive oil over the pesto. Then place saran wrap over it, pushing the saran down so that it's touching the oil. Finally, cover it. I'd suggest using an airtight container.

Blanch the basil (or arugula, etc) for 5-10 seconds in simmering salted water and quickly shock in ice water. Dry thoroughly and you're set.