Converter RGBA() to #HEX color with applying alpha-channel + additional opacity

rgba-to-hex.sass

// converter rgba(r, g, b, a) color to #HEX string without alpha channel,
// with optional applying afterwards opacity ($opacity)
// by default alpha channel for rgba-color is applying against white background,
// but you can change it by setting third argumnet ($background)
@function rgba-to-rgb($rgba, $opacity: 0, $background: #fff) {
  @if $opacity > 0 {
    @if $opacity < 1 {
      $opacity: $opacity * 100
    }
    @return mix(mix(rgb(red($rgba), green($rgba), blue($rgba)), $background, alpha($rgba) * 100%), rgb(255,255,255), $opacity)
  }
  @return mix(rgb(red($rgba), green($rgba), blue($rgba)), $background, alpha($rgba) * 100%)
}
// by this function you can calc next color composition to real colors:
// opacity: 0.12;
// background-color: rgba(0, 0, 0, 0.87);
p {
  $rgba-color: rgba(0, 0, 0, .87) ;
  color: rgba-to-rgb($rgba-color); // #212121
  // applying 12% opacity to that color
  background-color: rgba-to-rgb($rgba-color, 12); //E4E4E4 
  // is also possible to use .12
  background-color: rgba-to-rgb($rgba-color, .12); //E4E4E4 
}