Transform a color to its premultiplied version.
color-to-premultiplied transforms a color to its premultiplied version, which is needed when modifying images using Image Access.
must be a color-spec, such as the result of a call to make-rgb (see Color specs).
does not have an alpha component, it is returned without a change. If it does have alpha, it is transformed to RGB if needed, and premultiplied, returning a premultiplied RGB color.
You need to premultiply when setting pixels using Image Access in an image with alpha. The result is unconverted, so when using image-access-pixel it still needs to be converted (by convert-color).
CAPI User Guide and Reference Manual (Windows version) - 3 Aug 2017