Take the advice of the first reply, and look around for various techniques. The reason is that shadows are very much context-dependent. A shadow will pick up color from surroundings or from brightly lit areas, so no single technique works.
Also keep in mind that you do not necessarily want natural shadows. Color harmony with other parts of your painting are important.
That said, having myself looked around for advice from many others, I keep these in mind:
(1) Adding black is sometimes useful when the illuminated portion is green of blue-green. However, do not use white to make the green lighter; it should probably lean toward yellow in the lightest areas.
(2) If the illuminated portion is colorful yellow, then using a dull earth color is helpful for shaded areas. Darker shades can use any of a number of colors, since a colorful yellow will be strongly influenced by surroundings in shaded areas, and may not appear as a shade of yellow. Depending on the pigment yellow used for the bright areas (PY74 works for me), you may find that a touch of Dioxazine Purple is a mixing-complement solution, because the first result will be to dull and darken the yellow, which will eventually miss black by leaning to the brown side rather than the green side.
(3) If the illuminated portion is orange or red, consider a mixing complement.
(4) In a landscape, with large shadows illuminated by open blue sky, the shadows will have a blue cast. Alternatively, have the sun-illuminated areas slightly more yellow-orange than natural, so that the shadows look blue by comparison. But beware of excessive chroma (too garishly colorful).
(4) In the end, just do whatever looks good.
New artists, and this includes myself, often do not have the brushwork skills to do fine details. What looks good when finely detailed (as in a photo) may look garish when painted in lower resolution. So, one technique you can use (if you have a computer with image editing software) is to blur photos, so that you can see the masses of average color instead of fine details.
Last edited by Pinguino : 07-17-2017 at 12:45 PM.