Why not yellow?

it just didn't cannibalize itself.. you provide enough... the crispiness might mean you provided a tad too much. a green canopy at the end isn't bad on its own. No reason to cause a fade, if you don't want one. some hocus pocus out there that ignores basic biology may say differently. 12 weeks and still mostiyl white pistils and unripened calyxes? you have a long way to go. if crispy, lay off nutes for a bit.

By the looks in the picture your buds are not ready there are probably still two more weeks to go if not more I'm looking at the pistils and they look white and they're shooting to the sky they still need to turn brown orange or red and curl down in a crispy way after that the buds well becoming fatter and it will be obvious it is already done. Yellow leaves it is something that is achieved in late flowering when the plant does not have enough nitrogen and it starts to consume the lower bigger leaves first and then the rest in order to complete his life cycle but if nitrogen is added during the grow then you will not have yellow leaves only sporadically here and but not everywhere. hope this information helps you out good luck with your garden.

Hey :-) Look with a microscope 👌If the trichome heads are about 70% milky and 30% clear, you can start with the flush. If the trichomes are about 70% milky and 30% amber, it is a very good time to harvest 👍🙏🏻 but do not pay attention to the leaves of the trichomes on the buds 👍

Did you flush with pure water a week or two before getting ready to harvest? This makes me think it's not using up all the nutes, which might in turn effect the taste and or flavor of your dried product. Check out my first grow for tips tricks and to learn what not to do. Cheers and happy growing.