Good luck on the black - since it is back all the way through you will not get color contrast. As far as a color fill - I know of no way to keep from getting something that will stay put - even if you used a mask I don't know of a paint that will hold up to the wear. Something like rub and buff will go into the grains of the leather - not a good look. Brown and tan leathers do look good engraved the amount of contrast is determined by the way the hide was tanned and how much oil is in the leather - wide range of color contrast depending on these factors - but black is just black.
It is fun to engrave and I have done several projects with it - some things like the cheap leather billfold I carry did not have much contract but still looks cool when I show it to someone. Some of the hides I have bought have great contrast because of the tan process and oil added.

Good luck and have fun.