What does this Leaf color mean??

Das sind blattadern , kein Mangel nix. Möglich zuviel Stickstoff wegen dunklen blättern , aber keine Grund zur Sorge! Weitermachen!

Something is preventing the plant from creating chlorophyl. could be an iron, magnesium, or manganese deficiency that's gone on for so long the plant is triaging resources to stay alive. Also, with hemp/cannabis there are some insecticides and herbicides can cause yellow veining too, since these plants are phytoremediators that grab everything out of the soil--good or bad. Metals, toxins, poisons, etc.

Just part of the leaf

Not only colour, growing pattern and leaf form is already influenced by nitrogen toxicity. If you carry on, you'll have few solid buds, which will taste ugly, at least for first 3 month or so ...

I'm going to say nitrogen toxicity and the look of the newer growth has leaf production problems that could be mor down to root problems but could be a few things il leave that as my bit but another growmie might chime in with more info for you pal.. Good luck..