To touch up the scuffs on my Fuji's 40+ yr old Fujita Belt saddle, I use Kiwi leather dye. Fortunately, black is black... However, a trip to a cobbler (or if Tandy Leather is still around) may yield dyes in many other colors.

For water resistance, I try to treat the saddle -- including the undersides - with Pecard Silicone leather preservative. I also used this stuff on my leather shoes and boots when I was a mail carrier for 33 years. Applied once or twice a year and it kept my feet dry when walking through rain or snow and the leather never broke down. The 4 fl oz can has lasted me for well over 20 years!