Warum sind die Stängel von den Blättern halb lil...

Some natural purple will come as they flower. A lot of purple or red stems is magnesium deficiency. In your case I would stop giving so much nitrogen. You have nitrogen toxicity, and the excess nitrogen is gonna hinder your Cola development. In bulking the plants want mainly potassium and phosphorus light nitrogen.

That's probs jsut the led causing a slight burn, I wouldn't worry as its jsut on the stems at the very top. If the leaves start to go pale then I'd raise the light to stop it burning the plant.

as with most things - Maybe. if only at top, probably the intense LED lights. if not caused by lights or genetics, increasing phosphorous that you feed will help this. red petiols are a sign of low-p. red stems or streaks in stems are a sign of low S. stesms and petioles are 2 different things to keep compartmentalize in order to discern between the 2. leaves have petioles.

I wouldn't worry about it. It varies from one strain to the next.