SCSS mixin to flatten a color against another color

matte-color.scss

// ## matte
//
// Calculate the color created by flattened the input color against a base color with an opacity
//
// @param  {Color} $color: foreground color
// @param  {Color} [$matte]: background color (or `transparent`) to matte the foreground against
// @param  {Number} [$opacity]: mixing opacity of the foreground color, analogous to layer opacity
//         in an image editor
// @return {Color} the matted color
//

@function matte($color, $matte: #ffffff, $opacity: 1) {
    @if ($matte == transparent) {
        @return rgba($color, opacity($color) * $opacity);
    } @else {
        @return mix(rgba($color, 1), rgba($matte, 1), 100 * opacity($color) * $opacity);
    }
}