Purple tones in many indica lines come from anthocyanin that shows up when night temperatures drop into the low teens during the last few weeks of flower. Some strains go dark on their own regardless of temperature, while others stay green no matter what you do. Lines that colour reliably versus ones that only purple under stress are separated into two lists on https://kushindicaseeds.com, which saves buying seeds based on a single flashy photo. Colour says nothing about potency, though it does look nice in a jar.
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