Tips of buds turning black or purple?

I'm about 4 weeks into flowering outdoors. Strain is unknown. I've been feeling her big bloom every other watering. I'm just noticing some of the tips of the buds & also the tips of some of the leaves around the buds turning purplish. I'm not sure if this is normal and just a strain characteristic or if this is a deficiency. The temps in the day time are 65-75 nighttime goes as low as 50-55 at times It seems to be a dark purple to me but I could be wrong and it's actually black. Just wanted some opinions.

Bump

its the weather,cooler nights im guessing

vostok said:

2 my blog: https://www.rollitup.org/Journal/Entry/understanding-red-stems.30159/ tho is also a strain thing becoming more apparent

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Thanks for the article. I think it has to do with the weather getting cooler & like you said the strain starting to show it's true colors.