James Webb Space Telescope images are famous for their colour: glowing red dust, blue star clusters, golden galaxies. It is natural to assume those are the colours an astronaut would see, but Webb works differently. It observes infrared light, which is invisible to human eyes, and scientists assign colours to different infrared wavelengths so we can see the data. Those colours are not decoration: they encode distance, temperature, and composition.

Webb's targets are often extremely distant, and the light from the earliest galaxies has been stretched by the expansion of the universe into infrared wavelengths by the time it reaches us. Infrared light also passes through dust clouds that block visible light, letting Webb peer inside star-forming regions that older telescopes could not see. To do this, Webb's instruments must stay extremely cold, shielded from the Sun by a tennis-court-sized sunshield, because the telescope's own heat would otherwise swamp the faint infrared signals it is trying to detect.

To build a colour image, Webb observes the same scene through several filters, each capturing a narrow slice of infrared wavelengths. Image processors then map those slices onto visible colours, usually with the shortest wavelengths shown as blue, intermediate ones as green, and the longest as red. This is the same basic idea as a weather satellite image, where colours stand for data rather than for what the eye would see. When Hubble visible-light data is combined with Webb infrared data, the colour palette widens further, since the two telescopes cover different parts of the spectrum.

In Webb images, colour gives useful clues. Bluer galaxies in a combined image tend to be nearer and actively forming stars, while redder ones are often more distant, their light stretched further into the infrared, or they contain thick dust that absorbs bluer starlight. Within a star-forming cloud, red and orange usually mark cooler dust, while blue highlights hotter, younger stars. None of this is a precise measurement on its own, but across a whole image the colour patterns reveal structure: which regions are dusty, which are bursting with new stars, and which galaxies belong to the distant background.

The next time you open a Webb image, look for the colour story before the beauty. Find the blue knots of young stars, the red veils of dust, and the tiny red smudges that may be galaxies from the universe's first billion years. Remember that every colour was chosen to make invisible information visible, and that the image is a scientific dataset dressed for human eyes. That is what makes Webb images special: they are not just photographs of space, they are translations of light we could never otherwise see.

Webb is not the only telescope that uses assigned colours. X-ray and radio telescopes also produce images in wavelengths humans cannot see, and their images are coloured by the same principle: map invisible data onto visible colours so patterns become clear. Even everyday weather satellite images use false colour, with different colours standing for cloud temperature or vegetation health rather than for what the eye would see. Once you understand the idea, you will recognise it everywhere in science imagery.

The key question to ask of any such image is always the same: what does each colour stand for in this particular image? Reputable releases include a caption explaining the colour mapping, and reading it transforms the picture from decoration into data. With Webb images, that caption usually tells you which filters were used and what wavelengths they represent. A minute spent reading it will tell you more than an hour of admiring the colours alone.