Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.9)
// Compass (v1.0.1)
// ----

$primary-colors: (
  "red":   (rbga(255, 0, 0, .5), rbg(255, 0, 0)),
  "green": (rbga(255, 0, 0, .5), rbg(0, 255, 0)),
  "blue":  (rbga(255, 0, 0, .5), rbg(0, 0, 255))
);

// In my lizard brain, the following make perfect sense,
// but if used it returns and error:
// "List index is 2 but list is only 1 item long for `nth'".
// It won't see the values as a list. Bummer.
@mixin colors-w-fallbacks-2($color) {
  background: nth($color, 2);
}

// But when loopong the map, the values are seen as a 
// list.
@mixin colors-w-fallbacks($color) {
  @if not map-has-key($primary-colors, $color) {
    @warn "No color found for #{$color} in $primary-colors map. Ktnxbye.";
  }
  
  @each $color-name, $color-code in $primary-colors {
    @if $color-name == $color {
      background: "#{nth($color-code, 2)}";
      background: "#{nth($color-code, 1)}";
    }
  }
}

// Usage
.thingy {
  @include colors-w-fallbacks("red");
}

SassMeister-output.css

.thingy {
  background: "rbg(255, 0, 0)";
  background: "rbga(255, 0, 0, 0.5)";
}