I run a dehumidifier constantly in my grow room, In my opinion you couldnt get it low enough to be an issue, High humidity is bad because of mold and mildew issues.
Low humidity will generally just mean that your plants will transpire more water, and again in my opinion the more water going through your plants, the more nutrients that are being used, and that increases growth rates. Also its true when marijuana is in a low humidity environment the resin glands will actually produce more resins to protect the leafs from drying out.
Watering your plants will increase humidity in a grow room, and any grow room is generally going to have a rather high humidity anyway.
If you did need to increase humidity a pot of hot water will do the trick, but I dont think it would be a great idea.
If your worried about humidity levels get a humidity censor, as far as leafs being discolored chances are thats a nutrient problem, Posting some pics would help, but you should definitely check out these deficiency charts, look at all the pics find the one that looks like your babies.
GROWFAQ
Good luck
