Duotone Generator - Map a Photo to Two Colours

A duotone is not a colour filter laid over a photograph. The image is first reduced to luminance, a single brightness value per pixel, and then every one of those values is mapped onto a gradient running between two colours you choose. The darkest pixels become your shadow colour, the brightest become your highlight colour, and everything in between lands proportionally along the ramp.

Because the original hues are discarded entirely rather than tinted, the result is unusually cohesive. A set of photographs shot in wildly different lighting, indoors and out, warm and cool, will come out looking like a deliberate series once they all run through the same colour pair. That property is why the technique is a staple of album artwork, festival posters, event branding and streaming-service playlist covers, where a consistent visual identity has to hold across images the designer did not shoot.

The technique predates software by a long way. Duotone printing used two ink plates, typically black plus one colour, to extend the tonal range of a photograph beyond what a single ink could reproduce. The digital version keeps the look while freeing the colour choice entirely.

Contrast between your two colours does most of the work. A dark, saturated shadow against a pale, bright highlight keeps the image readable; two colours of similar brightness flatten it into mush regardless of how good the source photo was. Complementary pairs such as deep blue with warm orange, or the classic magenta and cyan, tend to read most strongly.