Definition

Bright / Dark
A bright timbre is one rich in overtones (Natural Harmonics)
         with a nasal, penetrating sound.
A dark timbre is one poor in overtones
         with a fluted, pure sound.
Each timbre confers different characteristics and possibilities on a given instrument,
         usually brighter in the lower register and gradually darker in the upper register.

Classification
Strings, in general, are moderately bright,
         with the brightest on the bottom, the basses
         and the least bright on top, the violins.
The woodwinds are a mixed bag -
         the flutes and clarinets are a dark pair,
         the oboes and bassoons are a bright pair.
                   In general, the brightest below (of lower pitch), the darkest above, a good disposition.
The saxophones are, like the strings, fairly close to center.
The horns are quite dark, perhaps the darkest instruments of the orchestra.
The brass are also a mixed bag -
         the trumpets are quite bright, especially with mutes,
         the trombones are medium in this spectrum,
         the tuba is quite dark, almost as much as the horn,
                   and does not really make a good bass for the section,
                             which, for all intents and purposes, is upside-down,
                                       with the darkest on the bottom and the brightest on top.

Usage

Foreground - Background
Bright sounds are perfect foreground material
         because they draw attention to themselves.
Dark sounds, on the contrary, are more discreet
                   and can accompany bright sounds very elegantly.
         Care should be taken when these dark sounds are given solo roles
                   that bright sounds do not overshadow them.

Legato / Staccato
In the winds, dark sounds have a better quality of legato
                   (flutes, clarinets, horns),
         and the bright sounds a better quality of staccato
                   (oboes, bassoons, trumpets).
However, the soft, round sound of dark staccato should not be underestimated,
         and certain examples, like the beginning of Mendelssohn's Italian Symphony,
                   are elegant proof of discriminate usage.
Here again, the saxophones are close to center.
In the strings, legato and staccato are produced by the bow,
         and are not really affected by the darkness or brightness of the sound.

Volume control
In the strings, volume is controlled mostly by the bow,
         and can be applied to any register of the instrument.
In the woodwinds -
         the dark instruments, flutes and clarinets,
                   control the volume better in the low than in the high registers, and
         the bright instruments, oboes and bassoons,
                   control the volume better in the high than in the low registers.
                             Check the corrections page for interesting precisions.
         The saxophones behave like the bright instruments, and
                   control the volume better in the high than in the low registers.
In the brass instruments, like in the dark woodwinds,
         volume control is difficult in the high register.