It is often asserted that value is the most important design element in a painting. To “demonstrate” this for yourself, notice that, if you take a black and white print of a “good” painting and a black and white print of a “so-so” or bad painting, you will see, usually, that the “good” painting has sharp differentiation of light and dark areas. This high contrast creates impact. You can even test this out with your own work. Take a photo of something you’ve worked on and get it reproduced in black and white. Does it have contrast? What could you do to improve it?
On the other hand, an artist can “break” this rule and effectively use “high key” or “low key” painting to create a mood. Key refers to the overall value of a painting. A high key painting is mostly pale, such as paintings of misty coastlines. Monet frequently painted in mostly pale colors. On the other hand, Ryder often painted night paintings which were mostly somber. This is called a low key painting. A painting can be low-key and high contrast or high-key and high contrast. For example, Ryder’s night paintings of the sea containing a moon were low key but high contrast.
The most limited tonal exercise (and fun to do), is to take black construction paper and tear it up into pieces of different sizes and shapes. Position these on a piece of white paper until you have something that is pleasing to you. (Don’t worry about this, please.) You can glue the pieces down with white glue or an acid-free glue stick (available from Office Depot.)
Look around for a landscape or still life and do a painting or drawing that is high-key and low contrast. Observe what you are doing to achieve this and write it down.