Value Studies are a great way of testing out ideas and compositions. I'm a big fan of using Prismacolor Illustration Markers (20%, 40%, and 60% Cool Grey) for doing quick thumbnail studies. But I've been on a gouache kick lately and I think it is a far better medium for this purpose.
With gouache you can layer extremely fast. You can also paint opaquely (markers are ink-based and function like watercolors - you have to work light-to-dark)
The following studies are just a few inches wide and can be completed in just a few minutes. Use a big brush and focus mostly on shapes and values. Don't worry too much about detail.
On this quick study of a rooty tree, I was able to layer white gouache for highlights on the twigs.
This quick concept ended up becoming a painting. See last week's post for the oil version (as well as some tips about painting greens).
Opaque white was used for the rock highlights and the clouds.
Opaque whites and greys were layered to create textures in the foreground.
Hopefully this will inspire you to get some gouache and try it out. Doing value studies is a great way to experiment and really nail down your composition before starting the oil painting. The more problems you can work out before you start painting, the better.