Colourise a monochrome image

The opening operation does not search for a source colour. Every non-transparent pixel is included, its luminance stays where it was, and only its chromatic axes move to the selected target. That makes a real one-colour treatment for scans, engravings and black-and-white portraits rather than a wide tolerance pretending to select the whole frame.

Choose the target swatch and inspect the darkest and lightest regions. A strong chroma may clip at the edges of sRGB; the preview and export use the same gamut reduction, so the file is the result you approved.

1. Original object colours are absent from a monochrome image and cannot be inferred by this arithmetic.
2. One hue cannot independently colour skin, clothing and a background; that needs masks or a trained model.
3. Colourising an already-colour image deliberately replaces all of its hues, so compare before saving.

Does this restore the original colours?

No. A monochrome file contains lightness, not the hues that produced it. This maps one chosen colour across that lightness structure and makes no claim to historical reconstruction.

Why do the highlights stay pale?

The colour operation keeps each pixel's lightness. A near-white highlight therefore remains near white, while mid-tones carry most of the chosen chroma and shadows stay dark.

Can I make a duotone?

Use the general remapper for a second operation if two parts can be selected by lightness, but this opening is deliberately a one-colour luminance map rather than a full gradient-map editor.

• Replace a colourThe dropper armed on open, two swatches, one operation and nothing else to manage.
• Change clothes colourA garment-first opening with the dropper armed and neutral fabric explained.
• Indexed PNG paletteRewrite PLTE colour definitions without touching compressed pixel indices.
• The general remapperHue shift, absolute replace and a stack of operations that can be reordered and edited.