Responses
They become colorful by becoming poisonous so when they deplete the color they are poisonless and can't hurt anyone with poison. So the zhen didn't eat anything to become posionless so that he could live and be in contact with other people, but mostly with the general.
Spoiler so if you don't want to know what happens quickly skip over this comment o//_//o
So from the raws it seems like Ran, the original master, was ill. I'm not exactly sure why :/ I'm thinking it was because he took care of the zhen and had to deal with countless poisons thus poisoning himself in order to create a beautiful zhen. Then after the current general (forgive me I forgot his name) finds the zhen in the mountain, he learns the truth about his brother's death. He also realizes the zhen refused to eat in order to deplete its color. Not really sure how/why but the zhen decides to lose its color so that they can "live" together. And in they end they return to their home. Feel free to correct me if I'm wrong but that's what I got from reading the raws and my broken Chinese :,0