Duotone uses the light and dark structure of a picture to create a controlled two colour treatment. It is more predictable than choosing a tint when you want shadows and highlights to follow separate palette colours.
Duotone Image lets you choose a shadow colour and a highlight colour. Each pixel's calculated luminance determines its position between them.
Start with black and white
With black shadows and white highlights, a pure red pixel becomes a grey near RGB 54, 54, 54. The weighted luminance places it below the midpoint even though its red channel is fully saturated.
Changing the shadow colour to a dark navy and the highlight to a pale pink creates a different palette while retaining the same underlying tonal progression.
These are mapped pixel values, not two separately printed inks or an ICC managed press separation. The name describes the visual treatment used by this tool.
Pick colours with enough separation
Two very similar colours can flatten the image and hide its structure. A dark and a light colour usually give more room for the subject's tones to remain recognisable.
The transform preserves source alpha. Save as PNG when the background must remain transparent rather than choosing JPEG and losing that flexibility.
This treatment deliberately changes colours. It is unsuitable as evidence of the actual colour of a product, uniform or document mark.
Keep an original and compare the effect at its final display size. For a website hero or social design, also check whether any overlaid text remains readable against both the shadow and highlight portions. A pleasing palette alone does not establish suitable text contrast.