Someone recently gave the tip to use washable markers. I went and bought some (pretty inexpensive, considering what good marking pens cost that are marketed for quilters) and tried various colors out on some fabric scraps. Works like a charm!
I found every color washed out beautifully and completely. My biggest concern was the yellow because it has been mentioned that the yellow doesn't come out completely. It did on my test patches, but when I go to use these on an actual quilt, I'm going to test them on scraps of fabric that are in that quilt just to be on the safe side.