A bruise is a contusion... damage to the capillaries where blood has flown underneath the skin. This causes the telltale black and blue look... bruises are actually most often reddish in appearance.
Likewise genitals vary in hue and darkness/lightness as far as genetics goes. This variance in coloration is normal and unique to our individual DNA.
This natural coloration variance is due to our individual levels of melanin. Some of us have more melanin than others. Melanin is drawn into the penis via vacuum pressure along with lymph & interstitial fluid.
An albino would be able to pump without experiencing ANY discoloration aside from bruising... they have no melanin and therefore their penis wouldn't experience any darkening.
Darkening that doesn't go away after a long break from pumping is 100% melanin. Capillaries heal themselves (it's what they do) quite quickly and any blood deposited under the skin drains away in a manner of days (or if a more severe contusion - weeks or even months) by severe contusion, I mean a car accident, street fight, industrial accident or a bad fall etc.
Most pumpers have to flat out deal with a dark penis... Scandinavian types or other very light skinned Europeans can pump without experiencing much change in color or darkness, but the rest of us are gonna get a dark fucking dick.
The upside is that you can bleach your genitals... if porn stars can bleach their buttholes and bleach their vulva/genital area,men can bleach their penis.
I am looking into this because I am "swarthy" yellow skinned and I was born with dark genitals before I ever started pumping. I also had a surgery as an infant that deposited a large amount of pigmentation near my belly button/lower abdomen... this pigmentation has been making it's way away from my belly and into my penis. Plus my ancestry is 9% Iberian peninsula and I actually produce a fairly large amount of melanin despite having reddish hair. I tan like a Brazilian.
So yeah, I am looking into genital bleaching myself.
Melanin can be reddish/brown in color so the OP really needs to distinguish whether or not they are experiencing a contusion or darkening from the depositing of melanin.