Insufficient data. Those who can afford a RMC J-24 are too well off to set any serious wear into it while those who could, ain't ever gonna drop $2K+ on a leather jacket.
There was this one member who wore the hell out of his Shinki horsehide J-24 by Diamond Dave so much that it indeed did turn full brown but that guy practically lived in it so yeah, I guess it technically might be possible.
Some, not all but some of the original Buco jacket's from the 50's were teacore to the extent they were not dyed through, you can see on the flesh side of zip tabs etc. These jackets are still black, only in high wear areas where the flesh comes through can you see the brown core. I'm nearly certain on that, someone far more informed than me can feel free to correct me though.
The Shinki McCoy's uses is dyed through far enough, the whole top layer in fact, that if you were to try and turn the jacket brown, you wouldn't really have much jacket left, it would just be flesh. I've seen a twenty year old, early McCoy's black teacore jacket that still looks as black as the day it was purchased.
Edit - I stand totally corrected, as displayed below. Lord knows how much wear and how long took that Buco to look like that.