Titanium is nicknamed “The Rainbow Metal” in some jeweler circles because it can be anodized in an electrolyte solution to produce a variety of colors. The colors are created by passing electricity through it to produce a light-refractive oxide on the metal’s surface. Watch this cycling rainbow effect happen on an over-denture bar in the video above.
It appears to travel across the anodized titanium color chart, from left to right.
Anodizing titanium generates an array of different colors without dyes, for which it is sometimes used in art, costume jewelry, body piercing jewelry and wedding rings. The color formed is dependent on the thickness of the oxide (which is determined by the anodizing voltage); it is caused by the interference of light reflecting off the oxide surface with light traveling through it and reflecting off the underlying metal surface.
Note that these videos were made by experts! In projects involving electricity, seek out the appropriate safety equipment and adult supervision.
Related reading: This titanium bike project at Instructables.