Talk to them, and you should tell them the truth.

There is no shift in mood in this sentence, it is already stated correctly.

Talk to them, and you should tell them the truth.

What is the shift in the mood in this sentence?

There is no shift in mood in this sentence. The mood is imperatve throughout, urging the listener to talk to someone and tell them the truth.

The shift in mood in the given sentence is from a command or suggestion tone to an explanation tone. To rewrite the sentence, it could be expressed as "Engage in open communication and honestly share the truth with them."