A preamplifier might load the microphone with low impedance, forcing the microphone to work harder and so change its tone quality. A preamplifier might add coloration by adding a different characteristic than the audio mixer's built-in preamplifiers.
A preamplifier could impose low impedance on the microphone, making it work more strenuously and consequently changing its tonal characteristics. Alternatively, a preamplifier might inject coloration by incorporating a feature distinct from the built-in preamplifiers of an audio mixer.