Working with watercolor often involves beginning with the lights and progressing toward the darks, which presents the challenge of needing to look past the darks to see what the subject would look like without them. Painting a quick study of the darks alone can help overcome this obstacle.
The bold graphic image of just the darks on a white background is quite memorable. Having isolated the darks once, it will be easier to peel them away from the lights and mid-values in your imagination, so that the first layers of a painting become clear.
You can also use the black/white study to discover the role the final layer plays in providing the narrative content of the scene. If the darks are revealed as the main source of the story the painting tells, you know you can be more casual with the earlier layers.
For homework, look for an image with strong darks and lights, like one of these, below, and then make a small, quick, darks-only study.
In each of these, it would be necessary to round the mid-values up or down. As a general guideline, if a shape is closer to white than to mid-value, make it white. If it's closer to black than mid-value, make it black. The study will reveal where greater subtlety is needed.
When you have finished the study, please do one of the following:
1) Dry the study thoroughly. Now quickly and loosely, paint the lights and mid-values right over the darks. Be efficient with your brushwork. If you go back and forth with your brush too much the darks will run.
If you've been having trouble making the first layers carefree, even when you've seen that it's the darks that will provide the definition, the exercise below ensures that your lights and middles will not be overly controlled.
2) On a separate piece of paper, make a study of the lights and middle values with no hard edges. Really. Wet the paper well (not dripping, but shiny), and paint quickly. When you intend to save whites, make the area you leave unpainted extra large, so the paint will not swallow it up as it diffuses.
When the wet on wet study is dry, apply the darks.