One of the worst things you can do, if you want your piece to look realistic, is to immediately reach for black when getting ready to paint your shadows.
Here's my painting, "Couple In Costume At Balboa Park.
I want to direct your attention to the blue overhang in the upper left hand corner.
For this particular piece, I used cerulean blue as the base color and ultramarine, mixed with black for the shadows. You can see it has obvious dimension. In the pic below, though, I've digitally colored the shadow on the side black.
Next, take a look at how I painted this table in my painting "Woman With Cabinet".
Here, I've painted the base of the table burnt sienna and for the shadow on the side, I used a burnt umber shade. Both shades were mixed with other colors, of course. Now here's that same table, with the shadow on the side digitally colored black.
Here's a graph of the mockups side by side.
Truthfully, when I paint a shadow, I usually just make it a darker version of the base color. Now, I ususlly mix black with the base color to darken it for this purpose, but that's not the same as actually painting the shadow black.
Thinking about shading for this post gave me the idea to try seeing how using complementary colors to shade would work. I painted six circles in different colors. Then, mixing each color with it's complement, I used that complementary color to paint a shadow on each circle. So, to put it more simply, I mixed orange with blue and painted a shadow on the orange circle, red with green and painted a shadow on the red circle, and so forth.
I wasn't crazy about the results, so I decided to try glazing some of the dominant color, so red over the green shadow on the red circle, and orange over the blue shadow on the orange circle, etc, to see if I could get any better results.
I think using a complementary color to shade works well when the complement is a cool color. Meaning that using blue to shade orange, green to shade red, and purple to shade yellow can work, but the reverse, orange to shade blue, red to shade green, and yellow to shade purple, does not. This is because shadows must always be cooler than the base color. That's because not as much light is hitting those areas and when less light is hitting an area, colors just appear cooler and grayer. I'm sure you've noticed if you've ever looked around a dark room, there's not as much light.
But I must emphasize that even using complementary colors that are cooler than the base to shade can only work if those colors are muted. No color will work as shadow if that color is bright.
But maybe you're thinking, okay, so I won't use black for shadows cast by objects, but what about using black for shadows cast by an object? Black is a perfect color for those right? Well, lets try this out with my painting "Cody The Dog.
In the image below, I have the painting as is, with the shadows cast by Cody's paws colored a darker purple and in one pic where I've colored the shadows cast by his front paws black.
I think you can see, even for shadows cast by objects, black is probably still not the best choice. It just doesn't look natural.
Painter of portraits and wildlife