This Side of Paradise - S1-E25
Corrected entry: Spock is wearing his normal blue uniform when shot with the "happy spores" from the strange plant. The next scene is with Kirk inquiring about Spock's location to which he is told nobody has seen him. Spock somehow manages to find some green overalls out in the meadows when we switch back to him. (00:17:40)
Correction: Remember, Leila took Spock to that location for the express reason of having him infected. She likely brought civilian clothes for him, knowing he would no longer want to wear his uniform.