Ah! You must be running A LOT of light....

From what I have read, (in aquarium plant books) plants will alter
thier pigment depending on the available light..... It's kind of like a
plant metabolism. This is also a very good indicator of your plant's
light requirements. I thought this was very cool stuff!!

Plants absorb a certain wavelength light (if I remember correctly) in
the blue-yellow range.

The darker green the pigment: The more light it will absorb thus
increasing photosynthesis. This possibly indcates the plant wants or
can use more light.

The lighter the pigment the less light is absorbed (more light is
reflected from the leaf) thus less photosynthesis is carried out. This
indicates the plant has an abundance of light available and is trying
to "cut back" on the photosynthesis.

Still other plants are able to generate a red pigment. This may result
in a red leaf tint to total red color..... The red pigment absorbs less
light in the useable color spectrum, thus limiting photosynthesis.

All this info come from:
"Encyclopedia of Aquarium Plants", by Peter Hiscock
It's full of a lot of neat scientific useful stuff just like this
topic.
Amazon link:
http://www.amazon.com/exec/obidos/tg...books&n=507846
Gee, wish I could get some $$ for the advertisement.......Fat chance.