It looks like a phosphorus deficiency.
A lack of phosphorous causes stunted growth and smaller leaves; leaves turn bluish green and blotches often appear. Stems, leaf stems, and main veins turn reddish purple starting on the leaf's underside.
The leaf tips of older leaves turn dark and curl downward.
Severely affected leaves develop large purpleish-black necrotic (dead) blotches. These leaves later become brozish-purple, dry, shrivel up, contort and fall off.
Also your plants should be pointing up towards the light in a V-shape. They look droopy to me.
Are you using nutrients? Is your soil organic? What type of light are they getting? Every little thing affacts your plants.
I HEAVILY recommend "Marijuana Horticulture: The Indoor/Outdoor Medical grower's Bible" by Jorge Cervantes. It has helped me through all of my growing problems.
