Nitrogen Deficiency?

One plant in particular is showing drastic color change on the bottom.

if you noticed it's just 1 plant out of the whole bunch which tends to happen when you have multiple plants sometimes. Keyword is sometimes. Did you do anything different recently? How far are your lights? Have you checked ph? What kind of water are you using? How is your feed schedule? Are you alternating nutes and water? Do you check ppms? What's your temps? What kind of lights do you use, how high are they? Etc.

K deficiency I think. Read about it. Good luck

Could be the start of Nitrogen deficiency but would need more info on the grow and maybe a full plant picture,

A full plant picture would be helpful. But I think it may be a simple case of overwatering.
Possibly Ph.