Good question. Chlorophyll is one type of
pigment that is part of the machinery of
photosynthesis, but there are other pigments that
are other colors that can do photosynthesis
In fact, even leaves that look green have
other pigments in them. You can see these
pigments when trees and shrubs lose their leaves
in fall (if you live where this happens). The
colors of fall leaves were in the leaves all
along, but hidden by the chlorophyll.
We see different wavelengths of light as color.
Sunlight has all the colors of the spectrum. When
light hits an object, some colors are absorbed,
others are reflected. When a leaf looks green,
all of the other color are being absorbed, but
green is bouncing back to your eyes.
Not all wavelengths of light have the same
amount of energy. Light at the purple end has
more. Light on the red end has less.
If you see an algae that looks red, what
colors is it absorbing, and what colors are being
reflected to your eyes?
If you are interested in questions like this,
you may want to study botany or plant ecology.
Thanks for asking,