If you learn how to fill your art with light and color you will be engaging in one of the most delightful aspects of creativity. Furthermore it is a way of adding depth and realism to any work of art.
One of the main ways to accomplish this is to decide on where in your work of art you will find the light source. The light source in painting is absolutely crucial to having things look real.
You start with the basics by painting in the rudimentary light and dark hues. The key is to paint in the darker hues in the places of the painting that are the furthest away from the light source. For instance, if your light source is from the right then the painting hues will be lighter on the right and darker on the left. This is also the key to painting shadows and making them look realistic.
Painting figures is similar except that you treat each human or animal limb (like an arm or leg) separately and paint darker hues according to where it makes the most sense.
Dark blue paint can be quite effective as a base for painting shadows. If you are painting with acrylic use one brush filled with acrylic paint to two with water. If you are painting light and color in oils use the same recipe but use paint thinner instead. This is the easiest way to add shadows to a painting without the risk of having the painting turn out too dark.
The shadows should also be in alignment with your light source. Shadows almost always run in a straight line from the light source to the figure and to where they fall. You can use a ruler to make sure this is visually accurate in your painting.
If you are painting a landscape then you need to be mindful of where the sun is on the horizon in your painting and make adjustments accordingly. This will help add atmosphere, depth and light to your painting.
Finally be sure to keep your darker paints thin so that you can easily remedy a mistake if you have to and note where the hues and shadows are so that your painting looks very realistic in the end.