Why might a person not be able to see coloured ink on a whiteboard?

Why might a person, who apparently isn't colour-blind, not be able to see red or green on a whiteboard? What is colour-blindness anyway? We try to find the answer in this Question of the Week. Plus, we ask which is worse for the environment - a few weeks of volcanic eruption or a few weeks of aviation. Like this podcast? Please help us by supporting the Naked Scientists