Another thought, darker green leaves can mean a couple of things - first is that the plant can compensate for too little light by making more chlorophyll, and second would be a response to a high-Nitrogen fertilizer. Neither is really any cause for concern.
I'd recommend gradually moving them towards more light if you can, or give them some extra light by hanging them near some fluorescent lights.
Do make sure that the pots are significantly heavier after you have watered, too. If not, the water is just flowing straight through, it's an indication that the potting medium has dried up and become a solid brick inside the pot. Not absorbing any water.
