Probably not lacking nutes.
Some strains will show color because it is in their genes.
Some will purple up when the temperature drops at night.

In your case, I'd guess that the soil chemistry is getting hard to manage.
When they get rootbound the ph can't be stabilized.

They're too big for their shoes but it's not too late to gently transplant.
Or, at least set that pot on top of a larger pot of soil and let their roots spread out a little.

Lookin' good for size and shape though. :thumbsup:

Aloha,
Weezard