Bob, welcome to the forum.
As others said, a marine varnish is best. But you are looking for something that doesn't exist, really. So "best" only means the process is slowed down a bit. I work quite a bit with Osage Orange and would just love to keep that bright orange/yellow color, but, alas, it is not possible. I have tried lots of things. All that will work to keep the color is to not expose your piece to light at all, keep in a dark vault.
"Folks is funny critters."
Think for yourselves and let others enjoy the privilege to do so, too. ~Voltaire