Reflected light is an important part of realistic painting. This is especially true when painting outdoors where direct light from the sun is so powerful. Bad landscape paintings have no reflected light or ridiculously exaggerated reflected light. No reflected light is a product of too much reliance on photographic reference material. The camera is lousy at capturing reflected light outdoors under most conditions. This is why photographs have overly black shadows. The range of light and shadow in most outdoor scenes is beyond the capabilities of most cameras. When painting from life it is important to be aware of the properties of reflected light and how they affect the scene you are looking at.
Anything that has direct light falling on it in a scene becomes a source of light itself.
You can prove this to yourself by going outside on a sunny day with a colorful object and placing it close to the shadow side of any other object, while the colorful object is still in sunlight. It will drastically alter the color of the shadow. If the object being lit is light enough and reflective enough it can even affect other objects in the sunlight.
Reflected light is never as strong as its source light
This is a problem for most beginners who tend to focus on color and are oblivious to value shifts. In most situations, reflected light belongs to the shadow and as such it must support, not compete with the lighted areas of your painting. While it can be effective to exaggerate the chroma of reflected light, raising its value too much will ruin the effect completely.
Reflected light is a combination of the local color of the object sending the light, the object receiving the light and the quality and color of the source light and ambient light in the scene.
As you can see once you have multiple sources of light like the sky and other objects, reflected light becomes a very unique phenomena dependant on all the other aspects of the scene. You can imagine that the combinations of colors these reflections will produce are not predictable. This is why direct observation and field studies are so important and can never be replaced by photographic reference alone.