That rainbow sheen on a laser-marked watch back or titanium knife isn’t paint, ink, or dye. There’s no pigment on the metal at all. The color is light interfering with itself. When a pulsed fiber laser sweeps across stainless steel or titanium, it isn’t cutting or burning the surface. It’s growing an ultra-thin transparent oxide layer, often just tens to a few hundred nanometers thick. Light then reflects off two surfaces at once: the top of that oxide film and the oxide-metal interface beneath it. Because the two reflected waves travel slightly different distances, they fall in and out of step, reinforcing some wavelengths and cancelling others. The survivor is the color your eye sees. This is thin-film interference, the same physics behind oil slicks, soap bubbles, and the blues on heat-treated steel. The catch is that oxide thickness sets the color, and thickness is set by heat. Grow the film thicker and the color marches through a fixed sequence, bronze to violet to blue to gold,...