When I’m painting a sunset/sunrise sky with a mountain, I have a tendency to make the sky gradients parallel the mountain ridge, even though that makes no sense.
When you negative-paint mountains, it’s just a lot easier to pull the paint along the edge of the mountain than create a straight line independent of the mountain. The problem is that life doesn’t work that way – there’s no reason for the sky’s color bands to parallel any mountain – so it can make the image ring false, or appear flat and lacking in depth, as if the sky is stacked on top of the mountain.
I learned a cool trick for dealing with that from this Youtube tutorial by Antonio Bartolo.
Bartolo paints the sky straight across his sketch, then picks up the paint on the rock face with a tissue while the paint is still wet. Because he’s not negative painting, he is easily able to maintain a simple, straight, horizontal gradient with a big brush.
To avoid this…
After negative-painting the mountain, sky gradient bands appear to be stacked on top of the mountain.
Try this…
Paint the gradient straight across the mountain.
While the paint is still wet, use a tissue to pick up the paint from the mountain.
You can see some of the possible downsides in my example. The tissue is a bit of a blunt instrument, so it picked up some paint outside the sketch. That can’t be immediately corrected, but I could incorporate it into the shape of the mountain. The tissue also doesn’t pick up the pigment 100% fully, some has already stained. I don’t think that’s much of an issue, though, because in painting the mountain it would blend into the shadows.