It's hard to tell from those pics and your description may need your npk and light source included.I have seen plants do this when not getting enough light....Like 20 watts per sq foot at that stage(30 being the min).You said your lights were at 5 inches so I might assume you're using flourescents which can cause this lightening of leaf color.You might have too low a ppm of nitrogen but only you'd know.Perhaps a little more info and we can help.