Safe color mixin

safe-color.scss

@function mix-alpha($color, $bg) {
    @return mix(opacify($color, 1), opacify($bg, 1), alpha($color) * 100);
}

@mixin safe-color($color, $bg) {
    background: mix-alpha($color, $bg);
    background: $color;
}

body {
  @include safe-color(rgba(255, 0, 0, 0.5), #000);
}

Output:
body { background: #7f0000; background: rgba(255, 0, 0, 0.5); }