“Starting with the pure pigment colour (directly from the tube), paints can be altered by:
• mixing with pure water or a white paint to raise the lightness up to the value of the paper, or step 10 on a value scale, to produce tints of the hue
• mixing with a black or dark paint to lower its lightness close to the darkest value possible in watercolours, around step 2 on a value scale, to produce shades of the hue
• mixing with another paint of similar value to bring its colour closer to gray without changing its value, to produce tones of the hue.”
In the case of shadows we are only interested in the shades although the tones may be of some use.
Shades. Paints can be darkened, usually by adding a dark neutralizing pigment such as neutral tint, Payne's gray, carbon black or synthetic black. This progressive darkening produces shades of the hue, and this blackening mixture also expands the value range to values below the value of the pure paint, and can take any colour all the way to black (that is, the dark gray that passes for black in watercolour paints). (Remember, no material paint can produce a pure black colour, and the darkest ‘black’ watercolours only attain a very dark gray value, around 2 on a value scale.)
Tones. Finally, the lightness of the paint can be held relatively constant, but shifted toward a neutral gray, by adding a mixing complimentary colour of higher lightness or a synthetic black diluted to the same value as the starting colour. (Mixing complements usually produce a neutral tone that is much darker than either complementary paint, so once a neutral mixture is reached it must be diluted up to the same value as either of the complementary colours.)
All the tones in the figure above have the same value as the original red, while the shades darken in value toward black. They may not seem to, but the inset figure, with colour information removed, verifies that the tones have a constant lightness. Compare carefully the "dullness" in the tones with the "darkness" in the shades. Try to imagine the colour difference with the pure red as carrying you toward either pure black or pure gray: this is the fundamental difference between a shade and a tone.
There are two types of shadow, form shadow and cast shadows. We can use the standard sphere and cylinder to demonstrate these as below.