Once you get multiple layers of canopy, orange, amber, and yellow start to become more important. These wavelengths penetrate better than red and blue because chlorophyll does not absorb them as well.
All the red is absorbed at the top canopy layer, and acts as a signal that it is not blocked by other leaves. When a lower branch receives yellow with the red filtered out, it knows it is being shaded and stretches until it sees the red again. The yellow also acts as a source of food for lower layers since all the red is filtered out at the top layer.
You should look up phytochromes to get a good idea of how shade avoidance works and this will all make a lot of sense.