The function color-from-premultiplied transforms a color, which is assumed to be premultiplied, to its un-premultiplied version.
color should be a color-spec (see Color specs).
If color is RGB with alpha it is transformed to its RGB un-premultiplied version. Otherwise color is returned without a change.
You get premultiplied colors when using Image Access, either by unconverting (using unconvert-color) the result of image-access-pixel, or by reading the values from the vector that is filled by image-access-pixels-from-bgra.
color-to-premultiplied
image-access-pixel
image-access-pixels-to-bgra
image-access-pixels-from-bgra
Image access
CAPI User Guide and Reference Manual (Windows version) - 3 Aug 2017