Why do some of my plant have dry leave or faded color?

here are picture of 1 of my nice plants and 1 that the leaves are kinda 2 tonned green is it normal?
http://img717.imageshack.us/i/img0047wl.jpg/
http://img200.imageshack.us/i/img0046tm.jpg/

i know im giving the right light needed and the right amount of sleep im doing 18 hrs of light 6 hours of sleep. I water the plants when the soil feels really dry. Is there anything else?

im not sure but it looks like you have a nute problem

is it very bad? because my friend he's the one who gave me it like that and grow a lot!