Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
Thankfully, players can randomize the colors as many times as they need to find one that works, but it is a troublesome rule to make work.
Source: https://gamerant.com/the-password-game-hardest-rules/