Why were the Martians green?

After centuries of trying to get plants to grow on Mars after it had lost its
atmosphere when its geo-magnetic shield was lost, they found that the Genes for
plant life were still in their DNA, but suppressed. They enabled the Genes for
the epidermal and their skin cells began producing chloroplasts and chlorophyll.
They would lie around on the surface sun-tanning all day. The cells harvested
the energy from the solar reaction to provide energy to their bodies. A side
effect of this therapy left them with green skin from the chlorophyll. Sadly,
without an ozone layer and geo-magnetic shield to protect them they all died
from melanomas and cancers brought on by radiation damaged DNA...