How do fireworks get their colours?

Most colours come from a particular chemical element. The colours in fireworks usually come from small pellets called pyrotechnic stars, which produce intense light when ignited. A star holds a fuel, an oxidizer that combines with the fuel to produce intense heat, a binder to hold the pellet together, and colour-producing salts: compounds of particular metals.

The heat does the rest. In a flame, the metal ions are excited thermally. Then each excited state relaxes back to its ground state by emitting a photon, a packet of light. The energy of that photon is characteristic of the element, so each element glows in its own colour. That is why the colour of a compound in a firework is the same as its colour in a simple flame test in a laboratory.

In 1786, Berthollet discovered that oxidations with potassium chlorate gave a violet emission. Later work showed that chlorates of barium, strontium, copper and sodium give intense, bright colours. In 1860, Bunsen and Kirchhoff saw unexpected sky-blue and dark red in their spectral emissions, which led to the discovery of two alkali metals: caesium (sky blue) and rubidium (dark red).

Not every metal that colours a flame suits a firework, though. Ideal colorants give a pure, intense colour at moderate concentration. And pollution is a concern because fireworks often contain heavy metals as the source of colour. The silver and gold trails of sparks follow different rules: they glow as incandescent solid particles, not through one chosen element.