First up on my list of "things to do so I don't think about losing my painting mojo but while still doing something art-related": experimenting with different color underpaintings or toned canvases. I've done a little experimenting in the past, but not with landscapes. I've seen some artists who have fantastic results with a red or crimson underpainting, and others with a warm, raw sienna underpainting. However, I wanted to try it to experience it myself. Lesson number one: don't just read about how to do something -- try it!
I toned some 6x9" gessoed sheets with alizarin crimson, cadmium red, raw sienna, and just for the heck of it -- black.I then proceeded to paint very simple quick studies of the same scene on each one, using mostly the same colors. And guess what? You really can tell the difference. Granted, these were very loose, quick studies, and I wasn't very careful about purposely leaving the tone come through. But I really could see differences.
Here is the Alizarin Crimson version, followed by a close-up.
And the black underpainting was as hideous as it sounded. I know some people who get very good results with this, particularly pastel artists, but it certainly didn't work in this particular experiment. I think there must be more to the use of it that I haven't learned -- or honestly, care to learn at the moment. Maybe I can put that farther down on my list.
All in all, I think the Alizarin Crimson and Cad Red versions seem to have the most potential. I've seen Bill Guffey use the red underpainting to great effect, especially in some of his Google Street View paintings (Link).
What about you? Do you use a toned canvas regularly? If so, what color and why?