Short answer
Because the lamps changed. Low-pressure sodium street lights emitted almost all their light in a narrow band near 589 nm, turning streets monochrome orange so that red cars and green hedges looked the same; high-pressure sodium was broader but still poor at showing colour; LED street lighting now shows colours far more accurately but, depending on its colour temperature and design, can add more blue light and glare to the night sky and to neighbouring windows.
For decades many streets in Britain and elsewhere were lit by low-pressure sodium lamps. They were very efficient, but they produced almost all their light at one wavelength, near 589 nanometres. Under monochromatic light, surface colours cannot be told apart: everything is some lightness of the lamp's own orange. A red car, a green car and a grey car differ only in how bright they are. High-pressure sodium lamps, which replaced many of them, emit a broader spectrum and give a warm, golden light in which colours are recognisable but distorted. Night-time colour, in other words, was set by lamp physics long before it was set by design.
White LED street lighting, now common, has a broad spectrum and can render colours much more faithfully, so façades, planting and clothing at night look closer to their daytime colours. It also made choices visible that sodium hid. A cool-white LED with a high correlated colour temperature has more blue in its spectrum than a warm-white one; blue light scatters more in the atmosphere, contributing more to sky glow, and many residents find cool-white street lighting harsh. Many authorities have moved towards warmer white LEDs as a compromise between colour rendering, efficiency and the character of the night-time street.
An observatory can filter out a single sodium wavelength almost completely. Broad-spectrum white light is much harder to remove from astronomical images.
Beyond street lighting, many cities floodlight bridges, churches and civic buildings, and some coordinate this through a lighting plan so that monuments are lit in a consistent, deliberate way. Coloured floodlighting can be spectacular, but it also overrides the building's own material colour, and a saturated wash that suits a festival can look garish every other night of the year. Warm white light usually flatters stone and brick; cool light suits glass and metal. These are aesthetic conventions rather than rules, and a coordinated plan exists partly to stop each building owner competing for attention with ever brighter and more colourful light.
The CIE's guide to obtrusive light, revised as CIE 150:2017, treats unwanted spill of light into windows, glare towards road users and upward light contributing to sky glow as environmental effects to be limited by zone — from dark rural areas to city centres. Colour enters that picture through the lamp's spectrum and through surface reflectance: light-coloured façades and paving reflect more light upwards than dark ones under the same lamps. A night-time colour strategy therefore involves lamp colour, fitting design and surface choices together, and it is one of the places where urban colour has measurable environmental consequences.
Why: High colour-temperature LEDs with bright, exposed emitters.
Fix: Choose warmer white LEDs, shielded fittings and appropriate light levels for the zone.
Why: Saturated coloured light chosen for events is left on permanently.
Fix: Use warm white as the default and reserve coloured lighting for occasions.
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
Sodium-vapour lamps produce light at a characteristic wavelength near 589 nm; low-pressure sodium lamps give only monochromatic yellow-orange light that prevents colours being distinguished, and high-pressure sodium lamps have a broader spectrum but still poor colour rendering.
Source: NIST Handbook of Basic Atomic Spectroscopic Data: strong lines of neutral sodium; Sodium-vapor lamp (Wikipedia)
FactStrong evidence
CIE 150:2017, the second edition of the CIE guide on limiting the effects of obtrusive light from outdoor lighting installations, revises and updates CIE 150:2003.
Colourwise interpretationModerate evidence
Replacing sodium street lighting with white LEDs makes surface colours at night much more recognisable, while cooler LED colour temperatures add more short-wavelength light that contributes to sky glow.
Based on: Follows from the narrow sodium emission spectrum compared with broad-spectrum white LEDs, and from the stronger atmospheric scattering of shorter wavelengths.
FactModerate evidence
Low-pressure sodium light is nearly monochromatic, which allowed observatories to filter the sodium wavelength out of observations.
Source: Sodium-vapor lamp (Wikipedia)
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.