I can't tell if that is just the leaf color or not. It looks darker like it could be bud rot but it has more of a purple hue to it, which is a fairly desirable trait in many cannabis strains. I don't see any obvious signs of caterpiller poop on the leaves. So, that would leave humidity and lack of airflow as a potential cause. How is that in your tent, fairly under control? Can you kind of open one of those buds with your fingers? If it is mushy and goopy, then its likely bud rot. But if it is firm and splits open, it might just be the color of the strain. Have you grown this one before?