Need help with yellowing/light green leaves please!

This is my first grow, I've been using this site to guide my grow and it has been going pretty well until this plant started turning yellow? Temp is around 85-90 (i have pretty good ventilation but it's a small space with a 400 watt MH) humididty is around 40. I'm using miracle grow organic soil (about 6-6-6). I'm just concerned about the color of the leaves. it's an ak-47 feminized plant. Haven't added any nutes yet, the plant is around 2 weeks or so. I know the picture isnt of great quality but the top small leaves are the color of the whole plant. downsize.jpg

any suggestions?
 
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