You're asking amateurs here, but here's what I remember.
Animals like us have no way of converting carbon dioxide to sugars
etc. All we can do is burn sugars etc and make use of the energy
released. Plants also burn sugar and make use of the energy released,
but in addition they can photosynthesize- use the energy of light to
convert carbon dioxide (and water) to higher-energy forms like sugar.
Plants are always converting sugars and oxygen into water and
carbon dioxide. They need to do this in order to grow, reproduce,
transport nutrients, and do other things. But in the presence of light
and the proper nutrients they will photosynthesize and make the
reaction go the other way too.
I'm just guessing, but since your plants don't have much carbon
dioxide they can't photosynthesize very much. Their green color comes
from chlorophyll, the key molecule in photosynthesis. If they can't use
much of it, they're probably programmed not to waste energy making very
much of it. That would leave them looking a bit yellow.
You might check this with a biologist to see if I got it somewhat wrong.
(published on 10/22/2007)