Colour temperature tells you what the light looks like. Colour rendering tells you whether the things under it look like themselves — and it is the figure usually missing from the box.
A surface has no colour of its own in any useful sense. It has a reflectance — a preference for returning some wavelengths and absorbing others — and what you see is that preference applied to whatever light is available. Give it light with a gap where its strongest reflectance lies and it has nothing to return there. The surface has not changed; the supply has.
Daylight and incandescent filaments both produce smooth, continuous spectra, so this rarely arose before. Many LEDs do not: they generate light by exciting phosphors with a blue emitter, and the result can be spiky, with real gaps. Colour rendering indices are an attempt to summarise how much that matters.
7 of the 22 contexts modelled here involve judging colour rather than merely seeing by it. In those, the rendering specification decides whether the work is right, and the colour temperature is a matter of preference.
Hair colour is judged, mixed and assessed here, and the client will next see it in daylight. Light that shifts tone means work approved in the salon and rejected at home. Deep reds are exactly where the averaged rendering index is weakest, which is why R9 is worth asking about specifically.
The tension. Salons often want a dramatic, warm interior, and the colour-assessment areas need something quite different. Treat them as separate lighting problems: atmosphere at the entrance and the wash stations, accuracy at the mirrors and the colour bar.
Small areas of highly saturated colour judged at close range, often against skin. Both the polish and the skin have to render faithfully, and the work is done at a fixed station, so task lighting can be specified precisely.
Makeup applied under warm light is routinely too heavy in daylight, because warm light hides exactly the tonal differences being corrected. Even, high-rendering light at close to daylight temperature, from the front rather than above, is the whole requirement.
Garments are bought on colour and returned when the colour is not what the customer thought. The fitting room is the critical point: it is where the decision is made, and it is very often the worst-lit space in the shop.
The tension. Shop-floor lighting is often specified for drama and fitting rooms are left with whatever was cheapest. That gets the priority exactly backwards.
Metals and stones are judged on subtle differences in tone and on how they catch light. Rendering quality decides whether a yellow gold reads as gold or as brass, and directionality decides whether a stone looks alive.
Work has to be seen as its maker intended, and a gallery is one of the few settings where the lighting specification is genuinely part of the exhibit. Conservation limits on light level often constrain the choice further.
A camera has no adaptation. Whatever the light does to a colour is recorded, and a spiky spectrum produces colour casts that cannot be fully corrected afterwards because the information was never captured.
8 of the modelled contexts want two incompatible things from their lighting, and the usual mistake is to resolve that with one specification rather than two circuits. A restaurant wants warm low light and food that looks appetising. A salon wants a dramatic interior and a colour bar where hair can be judged. A bedroom wants to feel restful and to have a mirror you can dress at. In each case the answer is to treat them as separate lighting problems in the same room, not to compromise on one figure.
Colour temperature and colour rendering are independent, and only one of them is usually on the box. Temperature decides what the light looks like; rendering decides whether the things under it look like themselves. A warm lamp that renders well is a pleasant room; a warm lamp that renders badly is the reason a grey wall turns green after dark. If a space involves judging colour — hair, makeup, fabric, food, paint — rendering is the specification that matters and temperature is a matter of taste.