Not to mention the psychological effect of a dim, reddish atmosphere replacing the sky humans are accustomed to. I'd wager the effect would be very depressing, over time.
Colonists could use energy to get artificial lighting to a decent level, but in a wholly artificial environment it might would be depressing to have no nature to experience. We evolved to like living in natural environments that we couldn't really reproduce on Mars. No oceans, no streams, no lakes, no time in the sun, no natural beauty, it sounds like a horrific nightmare to me.
