Yellowing is normal as they establish roots, dark spots are not. Do you have any pics to post? That would help quite a bit. MH is good for veg, but to flower you really want a HPS, even if you buy a conversion bulb for your MH. If you don't have either one then buy the "Bloom" CFL's that run at 2700 Kelvin. Google for Green Coast Hydro I think it is gchydro.com... they are good guys.