Hue/Saturation/Color/Luminosity blend modes swap perceptual dimensions between layers rather than mixing channel values

The Hue, Saturation, Color, and Luminosity blend modes work differently from all others: instead of operating per-channel on RGB values, they swap dimensions in a perceptual colour space (luma + chroma + hue) between layers. Hue: keeps base luma and chroma, takes top’s hue. Saturation: keeps base luma and hue, takes top’s chroma. Color: keeps base luma, takes top’s hue and chroma. Luminosity: keeps base hue and chroma, takes top’s luma. This makes them useful for colour correction (change hue without changing lightness) and sharpening (apply high-frequency luma contrast without changing colour). These modes are not invertible in general.

Examples

Luminosity mode for sharpening: apply a sharpened version of an image in Luminosity mode — only lightness is sharpened, no colour fringing. Color mode for colorising B&W: paint colour over greyscale, get coloured image with original tonal values.

Assessment

Explain why the Luminosity blend mode is used for image sharpening rather than Normal mode. What artefact does Normal mode introduce that Luminosity avoids?