DIMMEST

Not bright or colorful.

"The lighting was too dim for me to make out his facial features."

Not smart or intelligent.

"He may be a bit dim, but he's not stupid."

Indistinct, hazy or unclear.

"His vision grew dimmer as he aged."

Lessened, reduced

"The diminished Roman Empire never recovered from the sack of Rome."

Made to seem less important, impressive, or valuable

"She felt diminished by the report card."

Reduced by a semitone