I know what range I have to work from. Once its dry to the touch, I can unify specific areas with glazing if I need. The astounding realism that we see in some images is this approach taken to a very high degree of detail, where the tonal values are closely observed and finely drawn. I do my best to get the brushstroke down and then never have to touch it again. The color of the light dictates what the overall hue will be so the first decision I have to make is what time of day the scene takes place in or what kind of indoor lighting is present. As the drawing progresses, emphasis is placed on seeing specific half tone shapes and properly compressing them within their value group in order to achieve a unified drawing that properly represents the visual impression. Shade the darks, observing the shape and value, shading carefully up to the edge of adjoining light areas. Value is the darkness or lightness of a color. .
Often, in a drawing or painting, if the values are positioned correctly and are close to the reference in their tonality, then the illusion of form and light is achieved. Value drawing is like painting in graphite, and although the process is different to using a brush, you need to think in terms of areas as opposed to lines. Many artists use this step as their value stage, followed up by adding color with glazes. If I see an orange in the scene but only have blues and greens on my palette, I need to find the closest color I can get to orange on my wheel.
The next step is commonly called the grisaille a monochromatic value layer. When the time comes to start the final painting, I still have to prepare the canvas. The eye is easily fooled, and if I dont do this now, the drama of the painting may not be as strong in the end. Pay attention to drawing the shadows and background. An eraser can be used to lift off highlights. I use all kinds of color setups including monochromatic (one complimentary (opposite and analogous (similar) themes.