“If I am stumped by an area, I work on another spot; then, when I turn back to the trouble-some place, I’m usually able to see color.”
This speaks to eye fatigue. That is, the tendency of the rods and cones to need some time to recover. Generally, the recovery time is seconds, but I suppose that if you are painting an area of gray sky and it starts to look grayer and grayer, moving to some other area would help refresh the eye. Even more, it would let you think about something different for a while, which is helpful in any context when you are stuck.