The main stems are still green, although I can see signs of change, but some of the leaf stems near the top are showing reddening. I'm wondering if this is a nitrogen deficiency even though they've been well fed. They're not vegging anymore and have been under red CFL lights (500w) on a 12/12 cycle for the last week. Temperature is around 75-78. Humidity 40%. Ventilated and fanned. Himalyan Gold indica is the variety. The rest looks fine, leaves, etc, growing well, and looking healthy. Is this something I need to be worried about? here are some pics:
Generally if my leaves look healthy, I don't worry about getting a little color on my stems. It may just be a sign that you will have some purple buds soon. But, if you think your plant may be phosphorus deficient, you may want to feed some flowering nutes. Top dressing with a little high p guano would probably do the trick. And a little kelp never hurt anything. Those are my two most important products for flowering.
You can soak it in water to make a "tea". Maybe 1/4 cup of kelp per gallon of water. Let it soak for a day or so then water it in. Or you can just spread a few spoonfuls on top of the soil and water it in to each plant that way. If you make the tea, I'd take the leftover kelp after you soak it, and topdress with that. Don't waste anything. It will continue to break down and add nutrients to your soil over time.
If in doubt I foilar spray my kelp 1-2 times per week as a bonus, you can't really over feed using foilar spray, but then again red stems can be a strain / trait thing too