Warm and cool grays
Just as hues have cool and warm aspects, so grays can have cool and warm aspects. The following would be a useful exercise.
Do a monochromatic painting (black and white only) using warm and cool grays. There are many ways you can get these grays. You can mix a brown and blue pigment (such as burnt sienna and ultramarine blue). The proportion of brown to blue will determine whether it's a warm or cool grey. You can buy warm and cool grey markers at art supply stores. You can buy warm and cool grey pastels too. You can cut out warm and cool grays from magazines. Try a composition using warm and cool grays.
Seeing the light and color in shadows.
Beginners tend to use black to darken shadows. Black dulls and deadens color. That’s why I advocate leaving it off the palette. (This is not a hard and fast rule. There are times when a good sharp black is necessary.) But shadows are full of reflected light tones and can actually be richer in color than sunlit areas.
EXERCISE: Set up a still life or go find a landscape. Look for the reflected light in the shadows. Pay particular attention to the light in the shadows. This exercise should be done with an actual still life or landscape, not a photo. Photos are an interpretation of reality using the camera. They go through a printing process that “normalizes” and limits the color. (For instance, have you ever taken photos on holiday, amazed at the color and variety of the landscape around you, only to be disappointed later, thinking, “What did I see in that?”)
Composition with cool and warm colors
Try interpreting a still life, first using predominantly warm colors and then using predominantly cool colors.