You'd have to re-palm the gloves and use a dyed material and yes it is very difficult to do, you have to remove the bindings and cut all the threading, then clone the shape and re-sew everything back together. There are a few places that do this still, some are not so great at it and some are pretty good. CPR (custom pre repair) is the pro level for this but impossible to get stuff done if you're not a pro since they're so backed up with work for NHL clubs, or you can check out ITR on instagram/facebook, he does some great work and has about a 2ish month turn around and some fair pricing (just a heads up, this sort of work is not cheap, we're talking prob $80-100 for a simple re-palm, much more if you also want fancy new gussets or some other custom overlays).
https://www.instagram.com/itr_hockey/