I wrote this as a reply to something over on the botany forum and figured I'd be able to use it from time to time, so saved it. Let me know if you want anything explained a different way:

A higher degree of
expression of the pigment anthocyanin (which expresses the color red
or purple) in juvenile foliage is very common throughout the plant
kingdom for a couple of reasons. First, even though the pigment
absorbs light w/o being a part of the photosynthetic process, it
plays a photoprotective role for new leaves that emerge under high
light conditions ...... when they are in an ontogenetic stage that make
them more sensitive/vulnerable to light over-saturation. If you were
able to quantify it at home ..... the reduction in anthocyanin's
expression would be seen to correspond with a decreased need for
photoprotection, this, because photosynthetic maturation allows
leaves to utilize higher light intensities. A major consideration
(that occurs during photosynthetic maturation) is an increase in
chlorophyll, the pigment often referred to as 'nature's sunscreen for
plants'. In short, the red coloration disappears as photopigment
concentrations and leaf cuticle thickness increases, both of which
would be occurring as mesophyll underlying the leaf cuticle
differentiates into separate layers (pallisade and spongy).

Second (but probably not a consideration in F benjamina), the red
coloration is also very often associated with an increase in phenolic
compounds that serve as defense against insect predation or herbivory
by making leaves unpalatable. It's also thought in many circles that
the red coloration may make leaves more difficult for some insects
and herbivores to see, and may signal insects that the affected
leaves are a less nutritional food source than bright green foliage
elsewhere, or that it would make a poor receptacle for ovaposition (egg-laying).

Al