alloc_color(+Color, -Pixel)
     alloc_color(+Colormapable, +Color, -Pixel)
     alloc_color(+Colormapable, +Color, -Pixel, -Actual_color)
     alloc_color(+Colormapable, +Color, -Pixel, -Actual_color, -Exact_color)

Pixel is a (possibly newly allocated) pixel value for the colormap associated with Colormapable that is as close as possible to Color. Color is a color spec, as described in pxl-col-cs. Actual_color is the actual color of Pixel in Colormapable, and Exact_color is the color that should have been allocated (the same as Color, if it is a color/3 term). Colormapable defaults to the default screen (which means the color is allocated in the default screen's default colormap).