Sound is complex as it is usually composed of many sound waves with various frequencies. It is possible that when the sound is reverberated different frequencies are reflected with different amplitude. For example, the reverberated sound may accentuate the highs compared to the original sound.
The coloration of reverb is the change in relative amplitude of different frequencies in the reverberated sound compared to the original sound.
In simple words, the coloration of reverb is an equalization of the reverberated sound.
When reverb is created artificially some reverbs coloration would occur because of the way the reverb unit is constructed. Plate reverbs tend to equalize highs up (creating sibilance and shimmer), whereas spring reverbs may accentuate mid to low frequencies. In other reverbs (e.g., most digital reverbs) some coloration may be enforced by the user by sending the reverberated sound through an equalizer.