Ok, and I've figured out this dark greening too, I think. The whole grow I've had leaves curling up a bit at the edges - something I've been thinking has been a slight Mg deficiency. I've been watering with epsom salts in my water and, looking back, every time the plant has gone a dark green colour it has been after adding a good dose of epsom salts to try to correct these curling leaves.
Well, it seems that my plants aren't really Mg deficient and, since Mg is used to make chlorophyll, an excess can cause leaves to go really dark green. The curling, I'm thinking now, is probably being caused by the heat from the lights and the fan. I added the fan to cool the plants because the 400W HPS is quite hot and, being under my desk, I could only raise it up so high.
Picture today - see how close the light is?
The temps seem ok as it is, but to keep itself cool the plant has been having to transpire (sweat) heavily - for which it curls up the edges of the leaves to increase the surface area on the bottom of the leaves where the stomata are. This looks like Mg deficiency except that, critically, the leaves don't yellow like they do with Mg def.
Older picture - see how close the fan is?
At any rate, I've moved the fan a bit back now and the plant all along has been as far from the light as I could reasonably put it, but I think I'll just let her deal with it and see how everything fares. Seems to be going well so far, so maybe a bit of sweating isn't so bad. Watered with plain water today.