Purple stems means the leaf is a sink, green stems means the leaf is a source
purple stems tells us that the plant is mature (as does alternating nodes) and is preparing to flower, like a squirel storing nuts for winter, the plant starts storing nutes to make sure it has them to flower with.
Yet one more reason not to prune leaves
and don't confuse this with stems turning red which is the first sign of an N deficiency
This is the pix I got from
RM3
View attachment 2000387
I will post some pix of my plant later.
Edit**
Here you go, these are really good example's. Once you start seeing those stems turning red they are starting to store nutes to bloom. Once you start seeing that its important to make it rain at least once a week if you want to see her take off. Otherwise back off on the nutes and only feed ever other watering. It just depends on how aggressive you want to be. I made it rain a lot this time around and man, Im running out of space veritically and thats never happened before. I got one plant on the floor and its almost 4 feet tall. Chex my thread Im going to post some more pix tonight.
These pix are from the same plant different branches.
Sink:
Source: