Learning to see tone or value will help you create paintings that hold the viewer's interest. Tone is very much relative -- what is a dark tone in one context will appear lighter in another. It depends on the context.
When painting, get into the habit of squinting your eyes at your subject, which reduces the level of detail you see and emphasizes the light and dark areas. Mid-tones are harder to judge. Compare them to the adjacent tones in the subject and to the lightest or darkest tone. If you struggle with this, a monochrome filter will help you to distinguish tones or value in a subject.
If you struggle with tone or value, consider doing a value study before painting with color, or painting entirely in monochrome until you're more comfortable with tone or value. In his 7 Steps to a Successful Painting Brian Simons says: "If you get the values, you’ve got the painting."
