One of the unique features of the elk I photograph in Pennsylvania is their posteriors, which contrast beautifully with their darker hide and even darker neck and face.
Contrast is good in photography. Our human eyes see contrast immediately and what we see as an obvious contract is not always conveyed in the photographs we take. The problem is that the range of contrast depicted in photographs is not nearly as good as the range we see with our eyes.
Contrast is the difference between black and white and everything in between on a sliding scale. Contrasting colors make for a much punchier photograph. This is important to consider when composing a photographic subject. What is the background like? Is it similar or dissimilar from the subject? Part of the problem in photographing wildlife is that the subjects often blend in with their background too well. This is how God made them as a form of protection so they can hide from predators, but it makes our job much more difficult when trying to photograph these animals.
Contrast is good.