Here is a series of 4 practice pieces using burnt umber only. This helps me with value/tonal ranges, washes and various techniques such as layering, lifting out, scratching, masking fluid... etc. Very valuable exercise.
Even though they are just simple little paintings I learn a lot each time I do them.
The next series I'll do will be working with 2 colours, which helps with all of the above, plus mixing and blending. This is where I find the real challenge starts.