Why would the stems be purple and Woody i’m growing in promax HP in 3 L pots growing well but really skinny branches

The most common reason I see why people have purple stems is because the temperature is cold - usually during lights off. If your temp goes below 68F then you may see purpling stems. Also, if the water you're giving your plant is too cold then that can result in purple stems. Or it could be simply be genetics.