Most of the art students do not stop to think how to paint a shadow, they only add black to any color to darken it and obviously it is the logical way of thinking and it could not be said that it is wrong, but as a result many will see their works begin to become dark and the colors become dirty, what is happening is that the color black is destroying the color with which it mixes, is taking away its strength, turning it off, leaving our painting sad and sometimes with very dark shadows other times very dull and milky.

Advice: as a beginner you must restrict  yourselves the use of black and white colors to small amounts
Do not forget to follow this rule:
The shade of a color is the result of mixing this color with its complementary and adding a little bit of blue if you would like to darken even more, this is tested very easily for example by mixing the yellow color with the violet both complementary, the ratio of Each color will depend on how intense is the shadow of the object we are painting.
The result of adding a pinch of violet to yellow is an ocher, color that works very well for the shadows of any yellow object, likewise we can mix red with green and blue with orange, and also with colors not so pure , As for example if we have a green grainy we can add red to it and we will dark it slightly.

To understand this all it is necessary to study the Color Theory, to learn the basic mixtures, the mixtures between complementary colors, the effects produced by colors accompanied by others, the painting classes must have a separate course of color theory, which is one of the most complex courses in the art of painting and deserves all the necessary attention.

Another important factor to take into account is the color temperature, as long as the illuminated part receives a warm light the area in shade will be cool and if the illuminated zone receives a cool light then the shade will be warm, this Is fulfilled in nature, everything we observe complies with this rule, so if we respect this our art works will have shades with clean colors and the temperature will be appropriate to finish creating the effect of realism that we are looking for.
Applying glazes is the best way to control the color temperature, a glaze is transparent layer of oil paint thiwill result in a very smooth blend.