Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

@mixin rgback($color, $alpha: 1, $rest...) {
  @if 'color' != type-of($color) or 1 == alpha($color) {
    $color: rgba($color, $alpha, $rest...);
  }

  @if alpha($color) < 1 {
    background: url(
      '#{$RGBACK__PATH}' +
      '#{to-lower-case(str-slice(ie_hex_str($color), 4))}' +
      '-#{alpha($color)}.png'
    );
  }
  background: $color;
}
$RGBACK__PATH: '../img/rgba-'!default;

test {
  @include rgback(#F80);
  @include rgback(#F80, .5);
  @include rgback(255, 136, 0, .5);
  @include rgback(rgba(255, 136, 0, .5));
}

SassMeister-output.css

test {
  background: #ff8800;
  background: url("../img/rgba-ff8800-0.5.png");
  background: rgba(255, 136, 0, 0.5);
  background: url("../img/rgba-ff8800-0.5.png");
  background: rgba(255, 136, 0, 0.5);
  background: url("../img/rgba-ff8800-0.5.png");
  background: rgba(255, 136, 0, 0.5);
}