Each week my students fall asleep as they hear me say, yet again, "Keep your colors transparent as long as you possibly can!"
I'm a firm believer in the precept "keep your darks transparent, and load your lights". In my teaching, I try to get students to solve compositional and values issues while still in a transparent stage, before the temptation to color causes muddy mixtures as a consequence of muddy thinking.
Personally, I like to stay transparent as long as possible. This allows me to keep all my options open. An example is this grisaille that I did for my weekly plein air class yesterday. The canvas is 20"x20". This is a big canvas to attempt to take very far when one is also helping students at their easels. By leaving the hasty lay-in en grisaille, I could allow myself to think about the painting more later, when not in class.
Grisaille, strictly speaking, means "in grays', suggesting a nuanced black and white painting. In my loose usage, it means painting essentially in monochrome while keeping everything transparent. In a sense it's just drawing with the brush. With few exceptions, I do this with a mixture of burnt umber and ultramarine. For both I use Rembrandt colors because they are somewhat less-pigmented than some other brands and thus are easier to use transparently. The mixture varies during the grisaille, sometimes tending to the cool side and sometimes toward the warm. In this particular sketch I also used some W&N Prussian green (also transparent) for the grass and large shrub.
This morning, when I arrived at the studio, I immediately thought that I'd like to make yesterday's grisaille into a moonlit scene instead. I was able to completely change my conception of the painting because I'd done my lay-in just in values, essentially drawn in monochrome. If I'd hastily started slapping color on yesterday, under the noonday sun, I wouldn't have had today's option.
I put a quick covering of some homemade Payne's Gray-like color on the sky---just slapped it on. This particular mixture was just ivory black, ultramarine and OH mixed white. It has neither the usual tiny bit of red or of yellow ocher. For now I just wanted a cool, dark color.
I covered the rest with a very thin veil of pure, transparent ultramarine and then wiped off selected areas of the blue to become the "moonlight" on the building. Those bits of ultramarine that remained in the otherwise wiped areas became cast shadows from imaginary branches behind the viewer.
Here's a detail, probably close to actual size.
At this stage of the painting everything is, both literally and figuratively, still fluid. I could yet find a mid-day painting by giving it a different, lighter sky. But I think I'll cast my lot with moonlight.
Now all I have to do is correct the architecture, construct the trees, adjust the values, and paint the painting. Perhaps I'll do none of that, and just enjoy having had fun with it today.