How Colorants Work

A colorant does not add color to light - it takes some away. White light contains every visible wavelength; a material's molecules absorb the wavelengths whose energy matches their available electron transitions, and whatever is not absorbed bounces back to your eye. A leaf looks green because its chlorophyll greedily absorbs red and blue for photosynthesis and leaves green to reflect. The color you name is the complement of what was absorbed.

This is why color in materials is fundamentally subtractive - the same logic as CMYK ink, now at the level of single molecules. Understanding absorption is the key that unlocks pigment mixing, dye strength, and why some "colors" behave completely differently because they are not pigments at all.

Both pigments and dyes are colorants that work by absorption, but they differ physically, and the difference drives how they are used.

A colorant's identity is its absorption spectrum: which wavelengths it removes, and how strongly. Slide an absorption band across the spectrum below and watch the reflected color appear as the complement of the band you carved out.

Here is the classic paint-box puzzle, finally explained at the molecular level. Mixing pigments is subtractive: each pigment removes its own slice of the spectrum, and only what both let through survives. A "blue" (really cyan) pigment absorbs the red end; a yellow pigment absorbs the blue end. Mix them and red and blue are both gone - only the green middle is reflected. The mixture is darker than either parent, because together they absorb more.

How strong a color is depends on how much colorant the light passes through. The Beer-Lambert law says absorbance grows linearly with concentration and path length, while the light that gets through falls off exponentially. Double the dye and you do not double the darkness - you square the transmittance loss. It is why a drop of ink in water is pale, but a thick pour is nearly black.

Some of the most brilliant colors in nature contain no pigment at all. A soap bubble, a peacock feather, a morpho butterfly, an oil slick - their color comes from microscopic structure that makes light waves interfere. In a thin film, light reflecting off the top and bottom surfaces recombines; at thicknesses where the two are in step, a wavelength is reinforced, and where they cancel, it disappears. Because the path difference changes with viewing angle, the reinforced wavelength shifts as you tilt - producing iridescence, color that moves.

Absorption and structure cover most cases, but color has a few other origins worth knowing.

Six questions on absorption, mixing, concentration, and structure. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Colorant physics connects light, the spectrum, and subtractive reproduction. The numbers reflect each article's position in the editorial roadmap.