I’ve been wanting to anodize titanium and use the laser to create masks for the different colors. Translucent oxides form on titanium with heat or voltage. (I also tried lasering colors into niobium, which oxidizes similarly to titanium, but didn’t find the color band with the CO2 beam — just grey/black.) With both niobium and titanium, each volt corresponds to a thickness of oxide and refraction of a color. If you start at the highest voltage and work your way down to the lowest voltage for the colors you choose, theoretically, you can strip off the protective tape mask after each color is obtained and the thicker oxides (higher voltages) will be unaffected by lower voltage oxide layers. In essence, when anodizing with a tape mask, you are using a reductive method of printmaking like linoleum block printing, only the ink is an oxide layer.
DC voltage is applied to the surface of the titanium/niobium with a sponge clipped into an electrode. The ideal electrolyte to complete the circuit is phosphoric acid, so you wet the sponge with cola, or a solution of TSP, Cafiza, or Miracle-Gro. Two factors determine the oxide layer thickness: Voltage and time. If you vary the speed of your sponge with the electrolyte across the titanium, you’ll get a variety of colors due to incomplete oxidation at and below that voltage. It’s also possible to make a gradient by starting slowly and speeding up as you sponge across the metal.