Short answer
Each switches between two optical states in response to a trigger. Thermochromic inks mostly use leuco dyes in microcapsules that change colour when a solvent inside melts; photochromic lenses contain silver halide in glass or organic dyes in plastic that darken under ultraviolet and fade back when it is removed; electrochromic glass darkens when a small voltage drives ions into a tungsten oxide layer. All three are reversible, but all wear out — especially under ultraviolet.
Most colour-changing mugs, labels and novelty textiles use a three-part system sealed in microcapsules: a leuco dye (colourless in one form), a weak-acid developer, and a solvent such as a long-chain alcohol. When the solvent is solid, dye and developer are separated and the dye stays colourless; when it melts at the design temperature, they meet, the dye's ring opens and it becomes intensely coloured — or the reverse, depending on the formulation. The switch temperature is set by the solvent and usually happens over a few degrees. Such capsules are normally mixed with an ordinary pigment, so the change is between two colours rather than colour and nothing. Liquid-crystal thermochromics, used in strip thermometers, mood rings and precision indicators, instead change colour by selectively reflecting wavelengths that shift as their helical structure responds to temperature.
Photochromic glass was described by Armistead and Stookey at Corning in 1964: tiny silver halide crystals in the glass darken under ultraviolet as silver forms, and recombine when the light is removed. Silver chloride remains the classic inorganic photochromic, valued for resisting fatigue. Plastic lenses use organic photochromic dyes — spiro-oxazines and naphthopyrans — that open into a coloured form under UV and close again thermally. Because they switch by UV, most do little behind a car windscreen, which absorbs much of it. The dyes also switch more slowly in a rigid polymer than in solution, which is why lenses can take minutes to clear indoors, and they gradually lose their range over years of use.
Electrochromic devices darken when a voltage moves small ions — typically hydrogen or lithium — from a storage layer into a transition-metal oxide such as tungsten trioxide, changing its oxidation state so that it absorbs light. Because the tinted state persists, power is needed only to switch, not to hold. Self-dimming rear-view mirrors are the most common example; building and aircraft windows are the showpiece. The Colourwise interest is in colour: older electrochromic glass tends to look slightly yellow in its clear state and blue when tinted, and large panes darken from the edges inwards over seconds to tens of minutes, which affects both daylight colour in a room and how a façade reads from outside.
All chromic systems rely on a reversible chemical or physical change, and every cycle and every hour of ultraviolet costs something. Leuco-dye systems are damaged by UV, solvents and high temperatures; a study of commercial thermochromic prints found that UV exposure degraded both their colour and their ability to switch, with the printing substrate and ink type changing how fast. Reviews of thermochromic textiles list limited stability, weak wash durability and low colour sensitivity as open problems. For designers the practical rule is to treat chromic effects as having a lifespan — a season for a promotional label, a few years for lenses — and to test them under the light and temperature they will actually meet.
| Type | Trigger | Mechanism | Typical use | Main weakness |
|---|---|---|---|---|
| Leuco-dye thermochromic | Temperature | Dye meets developer when solvent melts | Mugs, labels, indicators, textiles | UV, heat and solvent damage |
| Liquid-crystal thermochromic | Temperature | Helical structure changes reflected colour | Thermometer strips | Narrow range, UV sensitivity |
| Silver halide photochromic | UV | Silver forms and recombines in glass | Glass lenses | Heavy; slower in some conditions |
| Organic photochromic | UV | Ring-opening dye in polymer | Plastic lenses | Gradual fatigue |
| Electrochromic | Voltage | Ion insertion into a metal oxide | Mirrors, smart windows | Slow switching in large panes; colour cast |
Why: Dishwasher heat and detergent, or UV, have damaged the leuco-dye capsules.
Fix: Hand-wash chromic items and keep them out of sunlight; the effect cannot be restored once lost.
Why: Windscreens absorb much of the ultraviolet that triggers the dyes.
Fix: Use dedicated sunglasses in the car, or lenses designed to respond to visible light.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Leuco dyes are the most important class of thermochromic compounds; they are typically microencapsulated with a weak acid and a solvent, and change colour when the solvent melts; UV, solvents and high temperatures shorten their lifespan.
Source: Leuco dye; Recent Advances in Reversible Thermochromic Materials for Smart Textiles: A Review (Materials, 2026)
FactModerate evidence
UV radiation degrades both the colour and the reversibility of commercial thermochromic prints, with substrate and ink type affecting the rate.
Caveat: Three commercial inks on two substrates.
FactModerate evidence
Photochromic silicate glasses sensitised by silver halides were described by Armistead and Stookey in 1964; silver chloride remains widely used in photochromic lenses, while plastic lenses use organic dyes such as naphthopyrans.
Source: Photochromic Silicate Glasses Sensitized by Silver Halides (Science, 1964); Photochromism
FactModerate evidence
Electrochromic windows change transmission when voltage drives ions into an oxide layer such as tungsten trioxide, hold their state without continuous power, and in older designs show a yellowish clear state and blue tinted state while darkening from the edges over seconds to 20–30 minutes.
Source: Electrochromism; Smart glass
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.