Leaf stems change color

It should be a lack of PK if there is too much N , but i don't think is this case (anyway the leaves are very green dark, so keep attention when fert) I think it's a simply genetics trait :) a lot of plant do that May be a stress like they said in comment below, but the stems should be all red, with all branches red too

Some stems turn red or purple in intense light and is normal. For others it suggests over feeding... If the stems that are lower on the plant and younger are green, you can be more confident it is the light thing. Otherwise you would have to rule out that it's not a genetic thing, as some strains have darker stems anyway, before looking at feed.

That seems to be the plant's defense mechanism against excessive UV rays. Good luck, friend

Red stems often are a sign of some sort of stress... but if it's just the top of the stem, I don't think I'd worry about it... the red usually starts at the base and works its way up to the leaf... Good luck!