It also could be brought about by the temperature, some strains, when the nighttime temps dip low enough, will change colors in various parts of the plant. Interestingly enough, that color change is because the plant can't take up a nutrient it needs, because of the cooler temperature. In that case though, it's normal, and the plant is fine.
I can't tell for certain, but it does look more like just normal plant genetics to me than a problem.
​​​​​​​What strain is it?