To a large degree, this is a consideration of aesthetic preference, but keep in mind that mulches that are dyed (usually black or red) can be of unknown and unreliable sources. Although most elements used to die mulch is free from poisonous chemicals, this is not a guarantee unless you know the source. Dyed mulches can also rob the soil of important nutrients that the mulch uses to break down over time, leaving the soil depleted of these elements (mostly nitrogen). And finally, mulches are often used as a moisture-retaining element. Dark or black dyed mulches heat up significantly more than natural mulch, and water therefore evaporates much more quickly, defeating one of the main purposes of mulching.