Converting electrical input to light
In a suitable semiconductor, injected electrons and holes recombine and emit photons. An LED converts part of its electrical input into light while other energy becomes losses and heat. Its mechanism differs from an incandescent filament, with electronic structure and carrier transport central to emission.
Materials and colour
Different semiconductor materials and structures yield different spectra, with an individual LED chip typically emitting a relatively narrow band. Band structure, recombination and packaging affect the output. Developing different colours required material-growth and fabrication advances rather than merely painting one universal emitting chip different colours.
Producing white light
White light can combine phosphor-converted emission with remaining LED light, mix several LED colours, or use hybrid arrangements. Spectral distribution affects colour rendering and colour temperature. Sources that both appear white can therefore differ in how they illuminate objects and reproduce their colours.
References: [1]
From blue emission to lighting systems
Efficient blue LEDs enabled efficient white-light sources, work recognized by the 2014 Nobel Prize. Complete lamps also need drivers, thermal management and optics, so lamp and chip efficiencies differ. Compact size and controllability support displays, indicators and other applications requiring carefully directed light.