Tan lines used to be a much bigger problem than they are now. All gloves used to have a big oval cutout formed by the fastening tabs. Also the back used to be very "porous" because it was crocheted. So you always has this speckled look on the your hands with a big lozenge shaped brown oval near the wrist. Then of course the big line between the wrists and the arms. Also there was much less use of sunscreen back then and the difference was very noticeable between hand and arm.

Now I find if I use SPF 50 or greater on both hands and arms, the differential is very small. Also if I use no sunscreen (not such a great idea, but...) when not cycling, the tan tends to even out because the hands aren't covered. And the gloves provide more complete coverage, so the funny tan tattoos don't form.

That is about as well as I know how to do. Have you thought of trying sun-free tanning cream on your hands to even things out. Could be a problem with the orange look though. You would have to be careful.