As window systems mature and energy-efficiency requirements tighten, Low-E glass has become a default specification. A question we hear regularly from building owners: why does my Low-E glass look a different colour from the neighbouring building? The answer usually lies in the coating system itself — and in the IGU build-up behind it. This article explains how Low-E coatings determine visual appearance and what to check before finalising a specification.
How Low-E coating colour is formed
The perceived colour of coated glass combines a reflected colour and a transmitted colour. By selecting coating materials and tuning layer thicknesses, manufacturers use optical interference to selectively reflect specific wavelengths of visible light — while keeping visible light transmittance (VLT) within the required range. This is how architects achieve a target facade colour without sacrificing daylighting.
Colour is measured under standard D65 illumination at normal incidence and expressed as L, a, b colour coordinates.
L, a, b colour coordinates
Low-E coatings are produced either by online (pyrolytic) deposition or offline vacuum magnetron sputtering. Base tones cover blue, green and grey families; varying thickness and material combinations can also produce neutral, gold (rose gold) and silver effects.
In practice, appearance is never determined by the coating alone. Incident light, transmitted light, sky background and the grey scale of the interior depth behind the glass all contribute, and perceived colour shifts with viewing angle.
Selecting a colour for the project
Beyond VLT and solar heat gain coefficient (SHGC), exterior colour and reflectance deserve deliberate attention.
Exterior reflectance. Outdoor reflectance of Low-E IGUs typically falls between 10% and 30%:
- Low reflectance (10–15%): minimal visual impact on observers.
- Medium reflectance (15–25%): best expresses the coating colour.
- High reflectance (25–30%): risk of glare and light pollution, particularly near urban arterial roads or in residential areas — best avoided.
China’s GB/T 18091-2015 (optical performance of glass curtain walls) addresses this directly: for curtain walls on buildings within 20 m of ground level alongside urban arterial roads, interchanges and elevated roads — and within 10 m on other road sections — visible light reflectance should be kept at or below 0.16.
Matching colour to building type. Typical practice: high-reflectance gold tones for banks and premium retail; colourless glass for libraries and exhibition halls; grey tones for commemorative buildings such as museums; and medium-to-high transmittance colourless, blue-grey or grey glass for residential projects.
The MGM project in Macau, using gold, silver and copper tones
Reflected vs transmitted colour. Looking out from inside, you see the transmitted colour. From outside, lower floors viewed more or less front-on show the interior, while upper floors — viewed at a shallow angle — are dominated by the reflected colour. Indoors, the coating’s reflected colour is usually imperceptible; it becomes visible only at night, with lights on inside and darkness outside, at specific viewing angles.
Chinese national standards specify colour-difference tolerances only for exterior reflected colour, not for transmitted or coating-surface colour. For conventional Low-E coating systems, transmitted colour varies little and generally stays within an acceptable range.
What shifts the colour
Substrate: standard vs low-iron float. Standard clear float contains iron and carries a slight green tint, which deepens with thickness and at shallow viewing angles. Low-iron glass (Fe2O3 below 0.015%) is essentially free of green. At equal thickness, low-iron glass has higher VLT, so a Low-E coating on a low-iron substrate delivers truer transmitted colour and a clearer view; edge colour is also purer.
Interlayer: PVB vs SGP. In laminated configurations, ionoplast interlayer (SGP) has a yellowness index and haze roughly one quarter that of PVB. PVB adds noticeable haze and a yellow-green cast, reducing clarity; SGP largely preserves the base glass colour with almost no change in see-through clarity.
Build-up and configuration. Colour consistency across a facade is affected by multiple factors:
- Different substrate types differ visibly; even the same type from different float lines can vary, and thicker glass reads darker.
- Different interlayers in laminated glass produce different colours.
- Adding IGU cavities increases the number of reflecting surfaces, raising reflectance and shifting reflected colour.
- Additional functional coatings — ceramic frit, anti-reflective, fire-rated or easy-clean coatings — all alter the visual result.
Colour comparison of Low-E coated glass at different viewing angles
In short: colour selection for Low-E glass should not be made from a swatch alone. Review full-size samples on site, at different times of day, viewing angles and weather conditions, before signing off on the final specification.