I wouldn't worry about slightly yellow sap if it tastes okay, the steam smells okay when you boil a test sample, and the syrup tastes okay when you finish that test sample. I wouldn't pull taps until all of your trees had quit running and the nights were always above freezing. And I would post pics of the trees here so the "experts" can tell you what kind of trees you have. If you can show the bark from a distance and from about two feet away, and also show the buds from about one foot away if you can, they'll tell you what to look for. I can only tell by looking at the mature leaves, but that's probably because I never see anything but reds around here.