I love the colors of anodized titanium, and I really enjoy making chainmail out of titanium rings I've anodized myself.
Anodizing is a process which uses electricity to produce a coating on the surface of a metal. Titanium is interesting because the process naturally produces different colours without adding extra dyes or pigments.
This instructable will show you how to color small pieces of titanium using readily available materials such as 9V batteries and Coke or baking soda. (It does not cover "painting" designs on large pieces of titanium.)
The results are not very predictable, but part of the fun is trying different things and seeing the results.
Step 1: Things you will need
Plain titanium rings. I got mine from http://www.theringlord.com/. Get quantities and sizes appropriate for the project you have in mind. I use larger rings as hooks to hang the small ones on during the anodizing process. You will need either some large titanium rings or a piece of titanium wire bent into a hook shape.
9V batteries. Three is about the minimum number to get nice colours. After taking the pictures for this instructable with three batteries, I tried using nine batteries. This produces a larger range of colours (and a more dramatic chemical reaction), and I highly recommend it. (It's also more dangerous, so be careful.)
Battery snaps. (See picture.) You'll need these (or some kind of battery holder) to connect the batteries together. You can get them at a store like RadioShack or anywhere that sells components for electronics projects.
An electrolyte solution. Lots of anodizing instructions mention Coke and Pepsi, and these do work. However, different soda formulas work differently (Coke Zero doesn't work at all). I've also had good results with grapefruit juice. For this instructable, I tried two different things: white vinegar and baking soda dissolved in water. Both worked, but the same number of batteries produced completely different colours. You may want to experiment with a few different things and see what works best for you. (Anything acidic is worth a try.) Keep in mind, though, that it will be easier to see what's happening if you use baking soda than if you use cola (because the cola is dark).
An empty margarine tub or similar container to hold your liquid.
Some scrap metal. This doesn't have to be titanium, but it needs to have an equal surface area to the rings you are anodizing. I used a large coil of thin steel wire, which worked for me.
Wire strippers and a small pair of pliers will make it easier to connect the wires. Also, if you have a multimeter you'll be able to check your circuit to make sure your connections are good.
Some tape to hold connections in place may help. You could also use test leads with alligator clips to connect the positive and negative ends of the circuit to the titanium and the scrap metal. (I didn't happen to have any, but it would be easier.)
You should really wear rubber gloves - I didn't, but I also shocked myself. Twice.