John, the only needles I have seen that are light in colour, somewhat yellowish, or even brown, are on dying/invaded trees where either aphids or pine beetles are cutting a swath down entire mountainsides as the pine beetles did in British Columbia a few years back. I would have to agree that yellow needles don't appear often either in my memory or when I drive around the province.
In my experience, a tree has pine needles that are all one colour if they are relatively healthy, EXCEPT in the spring when they produce new clumps of needles at the tips of all their branchlets. In that case, they are light green...again, only on the very tips of the branches.
I have a blue spruce, a twisted runt, that was beset by aphids every spring over the past eight years. It didn't thrive, but I was determined to make it a proud beauty. Starting in April, I would take a pressure washer and blast the undersides of the needles with the water. It drove off the aphids, and their herding and protecting ant masters. Of course I would have to return a week later and do the same thing until the season was over. Slowly, the spruce has begun to take hold, and is now showing signs of determination and growth. It's needles are a beautiful light teal, but at the moment, as I look, it's branchlet tips show the light breen bunches of emerging needles. I know it has aphids when those needles are bleached, browning, and somewhat yellowish in appearance.
