The colored holograms of Light are always blue as well.
One could always look at the possibility (however remote it is XD) that perhaps the hologram of Dr. Light... could possibly be Duo or at least some technology from Duo. If we work with the theory that Dr. Light used some of the same "good energy" tech from Duo, then wouldn't it also be possible in some sense that he managed to keep his "brain" alive by using some of Duo's technology as well?