After giving this some thought, I was thinking that it shouldn't be difficult to measure these terms quantitatively (but of course i'm not expecting anyone to do it this way).
It wouldn't be too hard to measure what the freq. response of a piece of music or test tones would be when recorded or when output from the the source (CD/DVD player). Then take the measurements again when the sound is output from the speakers.
Comparing the two graphs, bright could simply be when the high range output of the speakers is higher than the otuput on the actual recording. Dark would be excessive bass, or lack of high range from the speakers.
Just a thought ... but audiophiles tend to do this by ear and not by measurements, so it's anyone's guess what they really mean. Something that's bright to one person may sound dark to another, not too mention that most humans start losing their hearing in the upper range as they age, so presumably older reviewers would find speakers to be less bright simply because their hearing in the upper spectrum may be less sensitive.
I also don't buy this nonsense of combining a "bright amplifier" with "dark speakers" and vice-versa to achieve neutrality. IMHO the purpose of audio equipment is to faithfully reproduce the material as it was recorded, anything that tampers with this is adding noise to the system. Even if we may think it sounds better, it's still noise since it wasn't intended to be on the original recording.
In the end it all comes down to what sounds good to you, and what you enjoy listening to. Who cares what some reviewer says.