Vegetation contains chlorophyll, a substance that has an ability to convert water (taken up by the plant’s roots) with carbon dioxide (absorbed from the atmosphere by the plant’s leaves) into complex hydrocarbons in the form of sugars.
As chlorophyll is the reasoning as to why plants are green, for vegetation to reach its greenest form, it would have to be intaking large amounts of energy, which is absorbed from daylight.
So, simply vegetation is greenest when it is exposed to wide amounts of sun rays.