If they are darkening rather than yellowing I would be worried.
When lower leaves don't get enough light, they just kind of turn uniformly pale and yellowish.
When it is an N deficiency (normal in late flowering) they get yellow chlorotic patches and eventually become all yellow.

If they darken- do they shrivel too?- I would look to pH problems, nutrient burn, and/or heat stress.

No matter what the problem, they need more light to the low leaves. Flouros don't penetrate too far. To help this, you can give them some side light with CFLs or tubes.
Or you can just spring for an HPS!