Mike:
I think your last suggestion is most likely. Generally, we don't look closely at the penis unless something draws our attention to it. You don't get gangrene from sex, because it doesn't constrict the blood vessels. I've certainly never seen it. The gray color is probably a variant of normal which you've just noticed. varicose veins are quite common on the penis. If the skin is warm (normal skin color) you have nothing to worry about.
If you have pain, or are still in doubt, go to your doctor or a walk-in clinic. Don't be embarrassed. Reassurance is worth a lot.
best.
Dr. R