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.
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.