The intense green pigment of chlorophyll usually blocks out other colors such as the coveted purple. As the weather gets colder late in the season, the chlorophyll begins to break down allowing anothcyanin (purple) and carotenoid (yellow/reds) to show through.
It is very common for street dealers to call their stuff "purple haze" for obvious reasons. If they can get you intrested then you will probably buy it. Also, bud that is past it's prime is often darker in color and passed off as something special.