They can be due to either genetics OR a deficiency I believe. It's easy to tell the difference imo because one has a true purple look...dark (genetics) and the (potassium??? not positive) has a definite reddish tint to the stem....more like a fuschia if you will buy that.

One you've seen the difference you'll know. Having said as much.....I DO think he's got a fair N def going on there so it wouldn't surprise me if he also has a K def as well.