I use HF to etch titanium. It removes the anodization layer or whatever finish is on the titanium.
HF acid is some nasty nasty stuff so be sure to do some research - too big a splash will kill you in the most painful way if you don't have any calcium gluconate around, something about fluoride being attracted to the calcium in your bones and it anesthetizes the affected area so you won't feel it burning and symptoms arrive hours or days later.
You can find hydrofluoric acid at Walmart if you live in a state where it is not controlled. Look for Whink Rust Remover and make sure the back of the bottle says hydrofluoric acid. It can also be found in some auto parts shops as a rim or hubcap cleaner. These products are dilute concentrations of HF but is still extremely dangerous and even exposure to the fumes is deadly.
To etch titanium make sure that you completely clean the work piece of oils and grease with anything except for chlorine which has a nasty reaction with the HF. Get some Calgonate, plastic safety glasses, 2 plastic containers, a bucket, distilled water, baking soda, nitrile gloves, and some plastic straws or heavy nylon fishing line - HF will eat everything except plastic.
Put the gloves on (I use 2 on each hand) and then tie straws or the line to your oil-free workpieces but do not use metal wire only plastic or nylon. Fill one of the plastic containers with water and fill the bucket with the rest. Add a spoon or two of baking soda to that plastic container and dump the rest in the plastic bucket.
Put enough Whink solution to cover your workpiece and dip each, one at a time, in the solution, and watch the colors go "backwards" until the workpiece is a dull gray. Don't leave it in there after it turns gray, slowly pull it out and drop it in the plastic container with the water and baking soda. Repeat this until all your workpieces are etched.
Don't breathe the fumes. Don't splash the liquids. Pay very close attention to what you are doing.
Once you finish etching your workpieces, stick your gloved hands in the bucket and then dry them off. Now carefully take the acid container and slowly pour the acid in the bucket and finally put the plastic container in the bucket. What we are doing is neutralizing the acid for disposal. When the fizzing stops, it should be safe to dispose of, you can use ph strips to test. Check your local laws, you may need to dispose the stuff at a licensed facility otherwise I keep adding baking soda until ph is at 7 or higher and then put it down the drain. After your personal safety, ensuring this stuff is disposed correctly should be a priority. The bottle of Whink should also be kept in some kind of plastic bag or container and reasonably away from valuables, other chemicals, and children after using it. Make sure the cap is on tight.
If you are not going to immediately anodize, fill up another cup or a bag with water or rubbing alcohol and keep your workpieces submerged to avoid prolonged contact with air which will make the colors dull when you anodize.
Should any of this stuff get on you, immediately flush with water and then rub copious amounts of Calgonate on the area. What this does is tricks the fluoride into absorbing the calcium from the Calgonate instead of your bones. Then head on over to the emergency room and make sure that you tell them you spilled HF acid on you.
Have fun and try not to kill yourself.
P.S. Don't do this with aluminum unless you have a death wish.