You can finish with Tru Oil and then shoot clear high gloss Helmsman , which is exterior with ultra violet protection over the tips. Then light coat of Tru Oil over that to match balance of finish. When you rub it out just take it easy on the tips to not remove the Helmsman.
I found out the spit thing by taking a piece of purpleheart outside in the sun to see what it looked like. I spit on it to add a little moisture to see what it would look like finished. It immediately turned a dark purple color. Some kind of chemical reaction. It works. That's worked for me for many years. I have tried other things but nothing else works that I have tried. I now presand and shape my work with purple heart. Then do that and only lightly sand . I have bows that were built 15 years ago that have not changed from that beautiful dark purple.
God Bless, Steve