We started by talking about value, which reprents the light to dark range in art. Many, many artists recommend doing a value study when planning a painting. This is a monochromatic small study to lay out where the lights and darks will be. By extension, this leads into "notan," which is a Japanese design principle that applies to how the lights and darks are massed, or the composition. If I understand this correctly, and I may not--you can do a value study without notan. When you have reached a well-designed composition in your value study, then you have notan. Here's a description from Mitchell Albala on notan and another example from Diana Mize for Empty Easel.
By extension, you can use an color for a notan study, and once you've laid out the lightest and darkest values, you can start to add in mid-tone values. For watercolor, for the darkest value, I'll use a lot of paint and minimal water. For light, I'll preserve the white of the paper. And for a mid-tone in a monochromatic painting, I'll use more water with my paint. Here are some thumbnail sketches for value studies for a still life using raw umber with three values--dark, light, and middle tone. This study has three different interpretations--a mid/dark center of interest against light. A light/mid center of interest against dark, and finally a dark/light against mid. You can see, that how I choose to lay out the painting affects the mood and energy of the painting.
Then I need to extrapolate to color. This is where it gets tricky. I think I'll save another post for talking about the next step. In the meantime, though, here are two of my paintings that I tried to translate from the value study (which, hopefully has some notan). Both are limited palattes with prussian blue and indian yellow. The high key light background painting at the top of the page includes quinacridone orange (at this point, we'd eaten some of the pears, and we were down to one!). The dark background (below) painting includes quinacridone gold and raw umber.