The Color of Metals: Why Gold Is Gold and Silver Is a Mirror

A metal has no pigment. Its color - or lack of one - comes entirely from its reflectance spectrum: the fraction of light it bounces back at each wavelength. In most metals, a dense sea of free electrons reflects essentially all visible wavelengths nearly equally, and the reflectance curve is high and flat. That's why iron, chrome, and platinum are neutral gray mirrors - they play no favorites among colors.

Gold and copper are different because their electrons can do something extra: absorb specific photons in an interband transition, jumping from a filled band to the conduction band. This absorption kicks in above a threshold energy, which corresponds to a wavelength - so the reflectance curve has a step: low (absorbing) on the short-wavelength side, high (reflecting) on the long-wavelength side. Gold's step sits around 520 nm, so it drinks the blue and reflects the gold. Move that step and you move the color.

Pick a metal and watch its reflectance curve turn into its color. The demo integrates the curve against daylight and the eye's response - exactly how any surface's color is computed - so the swatch you see is what that spectrum really looks like. Silver's flat curve gives neutral; gold's step gives yellow; copper's later step gives red.

One control governs it all. Take a flat, silver-like curve and introduce a step - then slide where the step sits. With the edge far in the ultraviolet, the metal stays a neutral mirror. Bring the edge into the blue (~520 nm) and it turns gold. Push it toward green-yellow (~580 nm) and it becomes copper. One slider walks you across every warm metal there is.

Jewelers move that edge on purpose by alloying. Blend gold with copper and the edge slides redward - rose gold. Blend gold with silver (or palladium, nickel) and it slides toward neutral - white gold. Copper and zinc make brass, a paler yellow. The color is a smooth interpolation between the pure metals' spectra. Mix your own.

Because silver and aluminum reflect all wavelengths almost equally, they have no color of their own - they take on the color of whatever they reflect. A colored metal, by contrast, tints its reflection: gold makes the world warm, copper makes it ruddy. Send a scene into each metal and watch how the neutral ones stay true while the colored ones cast their hue over everything.

A quick field guide to the usual suspects and their reflectance stories.

Six questions on metallic reflectance, the interband edge, alloys, and neutral mirrors. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Metallic color sits alongside pigments, structural color, and how spectra become color - here's where to go next.