I'm a great believer in painting the basic background first, before anything else, especially with landscapes. It not only eliminates the stark white of a fresh canvas, but is far easier than doing it later. If you first paint the subject and then try to add the background around this, you have to work in a far tighter, controlled manner. It's also stressful -- the ever-present threat of accidentally painting over something.
This doesn't mean I paint a background once and then never add to it again. On the contrary, what I'm doing it's merely the first layer or two of the painting, a foundation of color/tone on which I build the landscape. In extreme cases, very little of the color put down at this is evident when I eventually down brushes and declare it done. What I'm showing you in this step-by-step demo is how to quickly and easily create a background that takes your painting from zero (white) to sixty (colors and tone) without wasting time or paint. It was done with acrylics, but the approach works with oils and watercolor too.
Photo 1: Pick the dominant color for your landscape. It might be green (for verdant landscapes), it might be a golden ochre (for desert landscapes), it might be blue (for a seascape), or something for a rocky landscape. You know what is going to feature in your landscape, and let this guide your color choice. If you can't decide, squint at the landscape or your reference photos and see what color dominates.
Now add two or three similar colors, variations on a theme as it were, plus white. In the example here, I've got quinacridone burnt orange as the dominant color, plus golden ochre, cadmium red, and cadmium orange. They're all strong colors, but by adding white and mixing the quinacridone orange into all of them, they'll work together as the background for a sun-drenched, mid-winter when the fields are all dried up, landscape.
When you've decided on your colors, establish where the horizon line is going to be. Or, put another way, how far down the canvas the sky is going to come. Mark this with a pencil, making sure the horizon is straight. Now take the darkest color from your selection and paint the horizon line. Thin the paint and work downwards.
That done, squeeze out some more of this color and all the others randomly across the canvas. Directly onto the canvas, with plenty of white. Don't stress where and how much, go with two blobs of each color and four of white, spacing them apart.
Photo 2: Grab a Big Brush This is where the fun starts. Grab a big brush, this isn't the time to be uptight and fiddle with a small one. Dip it into water or medium so it's nice and soggy, then slap it across onto the canvas. Brush vigorously, left to right, right to left, up and down, catching the dribbles as they happen.
Don't try to control the color mixing, focus on getting the paint spread across the whole canvas. I'm mostly using horizontal brushmarks in this example because it was going to be a landscape where I wanted a wide feeling to it. Having a base layer of horizontal marks will enhance this. Mark making should enhance the subject, not fight against or contradict it.
Don't focus on blending the colors neither, you want variation in how they're mixed. That's what creates the visual interest. If you're working with acrylics or watercolor, increase the time you've got to do this by spraying a mist of water on the canvas or paper before you start and again as you're working.