Purple indica lines like Granddaddy Purple descendants get their colour from anthocyanins, and in most of them the effect only shows when night temperatures drop in late flower. Some newer crosses turn purple regardless of temperature, which breeders now advertise as a selling point. Which strains colour up on their own and which need a cold snap is documented on https://kushindicaseeds.com with photos from several growers. Colour has no bearing on potency, so pick for it only if it matters to you.