It might be a sign of phosphorous deficiency. It usually shows up in flowering or during cold temperatures, because the cold inhibits phosphorous intake. It might also be the strain.
Cold temperatures (below 50F/10C) can make phosphorous absorption very troublesome for plants.Not enough Phosphorus causes slow growth and causes the plant to become weak. Fan leaves will show dark greenish/purplish and yellowish tones along with a dullish blue color to them. Sometimes the stems can be red, along with red petioles that can happen when having a Phosphorus deficiency. This isn’t a sure sure sign of you having one though, but can be a sign. Some strains just show the red petioles and stems from its genes.