Good question.
I think I disagree with Rar1's answer. Bright refers to an emphasis on the upper mid-range/treble. The sound has an unnatural glare to it. Dark refers to a diminished upper mid-range/treble. The sound is somewhat "shut-in" and doesn't sparkle when it should. Forward is more of a prominence in the mid-range. For instance a solo vocal recorded with the microphone inches from the performer's mouth should sound very upfront and present, but the same performer recorded in a mid to large size hall with more distant miking technique should sound the opposite. There's another phrase, warm, that describes an emphasis in the lower mid-range/upper bass.
