Vitamin K is used to clear up bruising from damage to the capillaries. I doubt it would irritate the skin on the penis unless you had a specific sensitivity. It’s often in ‘Under the Eye’ creams, which is some of the most sensitive skin we possess and the cream bases tend to be extremely mild and hypo-allergenic.
I’d suggest Arnica gel if your discoloration is from temporary, generalized bruising, as I suspect it would have broader benefits for your penile circulation overall. I don’t think a Vitamin K treatment would be bad, but I think it tends to help with more superficial capillary damage.
With regard to Tretinoin and Hydroquinone.
Tretinoin is basically pharmaceutical vitamin A and it can be a little intense. I’m sure you know that it is used for the treatment of severe acne. The less intense little cousin, Retinol Palmitate or Retinol, is probably more than sufficient to provide the Vitamin A your dick needs, but I’m not sure why it would need it. Unless you get pimples on your dick. It’s skin lightening properties are not great and probably misplaced in such an application.
Hydroquinone is in many over the counter skin lightening creams and is probably safe enough - again, variable sensitivities, your mileage may vary. It’s more for age spots or evening out discolored pigment in the skin; i.e. skin that is discolored for reasons other than bruising.
As to putting creams under your foreskin. Depends on how much and how often. Occasional applications will probably be fine, but I think the worry is having too much moisture and possibly developing yeast, fungus and the resultant skin rashes form same.
I don’t know if that is helpful to anyone, but there it is.
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God gave men both a penis and a brain, but unfortunately not enough blood supply to run both at the same time. - Robin Williams (: