What part of speech does the word warm function as in the expression "stay warm"?

Last week, as I was departing from work, a co-worker said, "Stay warm!" It was very cold that day. My question is how is the word warm functioning in this sentence? What part of speech is it? It appears to be modifying the word stay, but stay is a verb and warm is an adjective. My understanding is that adjectives cannot modify verbs. Please explain?

Solution 1:

The subject of the sentence Stay warm is an implied you. The adjective warm modifies the implied pronoun you. The verb stay serves as a linking verb connecting the subject with its modifier.

A similar construction is Be safe. The adjective safe modifies you (implied), linked by be.