"Riding to Jacks Canyon" 24x30, oil
When I start a painting, I typically block in my darks first. (Just as a refresher, most plein air painters start with four values in a landscape - dark, mid-dark, mid-light and light - as recommended by John Carlson.) However, if you make that darkest value too dark, you'll end up with a painting that's overall darker than it should be. By starting with the dark, you are keying your painting to that darkest value.
Of course, you can always lighten that dark later, but then you will need to readjust all your other values. And that's a lot of work!
And easier way to give your paintings a lighter feeling is to key off your lightest value. For example, in "Riding" (above) I first made the lightest possible mixture I could - a pale blue for the sky - and then darkened it just a tad. I let that be my lightest light, blocked it in, and then keyed all my other values to it.
If I'd started off the painting with darks, this painting, which has a great deal of shadow in it, would have been too dark.