Yes, there is a product called NO-SAN put out by Trio-dent,
Inc. Box 2082 Union, New Jersey 07083 908-686-1627
To etch _Titanium
Place resist covered piece in bath of NO-San full strength to
etch to desired depth revmove, rinse and clean off resist.
If you want both etch and color, do not remove resist, prior to
anodizing, etch and then anodise, then etch and reanodise to
Neutralize with baking soda and flush down drain.
I personallly have used it to remove surface oxidation. You
lose the shine off the surface of the niobium, but can repolish.
I have not tried to etch but know that it does remove the color
quickly , so I imagine that etching would not be too difficult.
Good luck, Let me know if you try this