Dimmer

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."

Disapproving, unfavorable: rarely used outside the phrase take a dim view of.

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

A rheostat that is used to vary the intensity of a domestic electric light

A switch used to select between the low and high headlamp beam on a road vehicle. (usually as "dimmer switch", primarily in North America; elsewhere "dipswitch" or "dipper switch")