I've finally followed Richard Schmid's advice and toned canvases with Turps instead of Odorless Mineral Spirits. Unless I'm painting a snow scene, heavy on the snow, I like to tone the canvas a predominant color. Others tone their canvas the complement of the predominant color and that is fine, especially for highly sunlit scenes.
The image above is from an OMS toned canvas, toned olive the day before painting (!) which I like for scenes heavy with foliage. It takes forever to dry, like days. A La Prima: this makes it necessary to lay on think paint especially on light skies, since laying on thin paint mearly muddles the desired sky color if the toned canvas isn't completely dry. You can see I used thick sky color just laying it on, no swishing the brush.
By contrast, this image shows a canvas toned with turps wash of olive in the morning and painted over the same day. No muddling of the sky color: Titanium White and Manganese Blue. The Turps stinks to me, so I lay on the wash outdoors. It drys so fast there is no stink to the canvas. Voila!
PS In each case I wiped out the sky area after laying on the wash.
