Different people - notably those with dark or very dark skin - have different degrees of pigmentation in "transitional" areas such as the lips, inside eyelids and of course at the other end, the genitals. This would be particular apparent in a woman's vulva.
In particular, pigmentation does often relate to the degree of exposure - but not always.
Your foreskin is clearly such a "transitional" area, the inside foreskin may variously be as dark as the outside, or quite pink. Much like your mouth.
If you have never been able to retract, than you should have little concern about any form of skin disease and can reasonably conclude that these colour variations of areas that have not previously been exposed, are perfectly normal.
You have some stretching to do. Read a lot of the previous material here and get working on both forms of "exercises". Don't worry about retracting for the moment, just give it a gentle try every week or two (or more often if you can't resist) to see if you can detect any improvement but don't get upset if you don't. Just do the work.
Note that it will take time, but you have no particular need to hurry. You can do everything you need, sexually, just as it is.
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