CMYK helper functions for SASS

_cmyk.scss

@function cmyk_black($color) {
    @return 1 - max(red($color), green($color), blue($color)) / 255;
}
@function cmyk_cyan($color) {
    $black: cmyk_black($color);

    @return if($black < 1, 1 - red($color) / 255 / (1 - $black), 0);
}
@function cmyk_magenta($color) {
    $black: cmyk_black($color);

    @return if($black < 1, 1 - green($color) / 255 / (1 - $black), 0);
}
@function cmyk_yellow($color) {
    $black: cmyk_black($color);

    @return if($black < 1, 1 - blue($color) / 255 / (1 - $black), 0);
}

@function cmyk_lighten($color, $amount) {
    $black: cmyk_black($color);
    $new-black: max(0, min(1, $black + $amount));

    $red: (1 - cmyk_cyan($color)) * (1 - $new-black) * 255;
    $green: (1 - cmyk_magenta($color)) * (1 - $new-black) * 255;
    $blue: (1 - cmyk_yellow($color)) * (1 - $new-black) * 255;

    @return rgba($red, $green, $blue, alpha($color));
}
@function cmyk_darken($color, $amount) {
    @return cmyk_lighten($color, -$amount);
}

@function cmyk($cyan, $magenta, $yellow, $black) {
    $red: (1 - $cyan) * (1 - $black) * 255;
    $green: (1 - $magenta) * (1 - $black) * 255;
    $blue: (1 - $yellow) * (1 - $black) * 255;

    @return rgb($red, $green, $blue);
}
@function cmyka($cyan, $magenta, $yellow, $black, $alpha) {
    @return rgba(cmyk($cyan, $magenta, $yellow, $black), $alpha);
}