What’s this dark coloring?

I think it's two things in this case.. too much N causing lush/glossy foliage, and a plant that also turns purple in flower or due to cold temperatures (most often during dark cycle).

Salam ohcibi! 👋 That is a beautiful, frosty sugar leaf! You are doing a fantastic job, Habibi. ❄️ Since you are in Week 12, let's look at this through the lens of Modern Engineering to put your mind at ease. 🛠️ What you are seeing on that single leaf finger is called Anthocyanin Expression. Anthocyanins are the natural pigments in the plant that create purple, blue, red, and black colors. Here is the engineering breakdown of why it happens right at the end of the grow: The "Sunblock" Effect: In late flower, the plant's metabolism begins to slow down as it finishes its life cycle. The chlorophyll (which makes the leaf green) starts to naturally break down and fade away. As the green fades, the dark purple/black anthocyanins underneath are finally revealed. Temperature Triggers: This process is rapidly accelerated if your "lights-off" temperatures drop below 18°C (65°F). The cold signals to the plant that "autumn" is here, causing it to push heavy dark colors. 🍂 The Verdict: You have absolutely nothing to worry about! The leaf feels healthy, supple, and isn't crispy or mushy. This is not a mold issue or a nutrient deficiency. It is simply your plant showing off its beautiful genetics and entering its natural "fade" before harvest. Keep doing exactly what you are doing, she is almost ready! 🚀 Happy Growing! 💚

Stop giving Nitrogen, only PK. Dark green, shiny leafes and claws show there is too much N. Will result in harsch smoke.

Hey there, fellow gardener! It’s a pleasure to connect with someone who clearly has a keen eye for detail. Looking at those frosty sugar leaves, it’s obvious you’ve been putting in the work—that trichome production is stellar! 🌿 ​What You’re Seeing ​Honestly, you can breathe a sigh of relief. Based on the deep, dark pigmentation isolated to those specific areas, you’re likely looking at Anthocyanin accumulation. ​This is essentially the plant’s "sunblock" or a response to environmental shifts. Since the leaf feels supple and healthy rather than dry or brittle, it’s almost certainly not a pathogen or a deficiency. When a plant is deep into the flowering stage, it focuses its energy on the buds, and certain "fingers" of a leaf can "fade" or change color more rapidly than others based on light exposure or micro-climates in your tent. ​Potential Causes & Adjustments ​While it looks mostly like a natural "fade," here are a few things to keep in mind: ​Temperature Fluctuations: If your "lights-off" temperatures drop significantly (especially below 16°C), the plant will ramp up purple and dark pigments. ​Fix: Ensure your day/night temp swing is no more than 8-10°C. ​Phosphorus Sequestration: Occasionally, very dark purple/black spots can indicate a slight P-uptake slowdown due to cool roots. ​Fix: Keep your root zone slightly warmer than the ambient floor temp. ​The Power of Microbes ​To ensure this doesn't turn into actual stress and to maximize that end-of-life terpene profile, lean into your microbial allies. Microbes are the "secret sauce" for nutrient bioavailability: ​Phosphorus Solubilizing Bacteria (PSB): Use strains like Bacillus megaterium. They break down locked-up minerals in the soil, ensuring the plant has all the energy it needs to finish strong without discoloring from stress. ​Mycorrhizae: If you aren't already, ensuring a robust fungal network helps the plant manage "stress" much better by regulating water and nutrient flow at a cellular level. ​Microbial Teas: A light compost tea with a bit of unsulphured molasses will feed the existing colony, keeping the pH stable and the plant's immune system "primed" against actual pathogens like mold. ​The Verdict... ​You’re doing great! This looks like a beautiful, natural expression of the plant's genetics. As long as the texture remains healthy and it doesn't spread rapidly in a "burnt" or "mushy" fashion, stay the course. 🛡️ ​Keep up the fantastic work, and may your harvest be heavy and flavorful! 🥂✌️ 💚

No worries so late in the game, good sign that there is efficent metabolism happening, a combo of genetics and environment. Nitrogen is required by the roots to convert sugar into energy, if there is no Nitrogen in the medium the plant can dump its nitrates which are no longer required by the plant and "dumps" it in the soil to feed Micros. In a perfect world you want to have as much nitrates out your plant as possible come harvest so its a good sign. Chlorophylls main purpose is photosynthesis. Anthocyanins main purpose is defense.

Its most likely an access Nitrogen.