This is a very important thing to learn if you want to make your art realistic. This is why I try to never use the dodge and burn tools, since they just darken or lighten the colors, when more realistic highlights and shadows change around the saturation and the tone of the colors, not just how light or dark a color is. One of the big rules of art is that if you're going to darken a color, don't just do it by adding black, and if you're going to lighten a color, don't just add white.
That kind of strayed off of what the original point was, though. Which is that yes, art looks flat and boring and unrealistic if it's all one color, just at different values. Something that you should be aware of for more detailed art pieces is that light reflects off of objects onto their surroundings, which changes the color of the surroundings. If you put a bright red box into a blue room, there'd be a bit of red on the walls of the room. Similarly, if you had a character wearing a green scarf, a little bit of green would reflect off of the scarf onto the edge of their chin, if the light was going in the right direction. This is easy to overdue, and you probably won't want to do this for quicker, simpler color jobs, but just a little hint of the surrounding color can add a lot to the realism of a piece.