i would have to say a red/purple stem is high stress caused by pH or genetics(some strains have purple stems).
Yellowing of Younger leaves............. Fe, Mn.
Yellowing of Middle leaves................ Mo.
Yellowing of Older leaves................. N, K, Mg, Zn.
Yellowing Between veins................. Mg, Mn.
Old leaves drop................................ N.
Leaf Curl Over.................................. Mg.
Leaf Curl Under................................ K, Cu, Over Fert.
Leaf tips burn, Younger leaves........ B.
Leaf tips burn, Older leaves............. N, Zn.
Young leaves wrinkle and curl.......... K, Zn, B, Mo.
Dead areas in the leaves................. K, Mg, Fe, Zn, Mn.
Leaf growth stunted........................ N, P.
Dark green/purplish leaves and stems......... P.
Pale green leaf color..................................... N, Mo.
Leaf Spotting.......................................... ...... Zn.
Spindly........................................... ............... N.
Soft stems............................................. ....... N, K.
Hard/brittle stems........................................ P, K.
Growing tips die........................................... K.
Stunted root growth.................................... P.
Wilting........................................... ............... Cu.
Use this chart as a reference ONLY. This is not a guaranteed diagnosis of your plant and you should only use this as a guideline to help you figure out what your nutrient solution is lacking...
some leaves are purple because of the strain and strain only. i see it as stress and big lead my plants about 10-7 days before i harvest. to ensure the middle and lower buds get light.