The converted color color-rep is a color representation like that returned by convert-color. If needed, color-rep can be converted to an RGB value using unconvert-color. color-rep can contain an alpha value, for images with an alpha channel, and in that case the values are premultiplied.
image-access must be an Image Access object returned by make-image-access.
If the result of
image-access-pixel on an image with alpha is used elsewhere (for example drawing a string with the same color), to get the same color you need to un-premultiply it first using color-from-premultiplied. When setting the color that came from elsewhere in an image with alpha, you will need to premultiply it using color-to-premultiplied. For images without alpha, premultiplication has no effect.
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017