rainbow.scss

@function rainbow($position, $colors) {
  $colors: if(type-of($colors) != 'list', compact($colors), $colors);
  $gradient: compact();
  $width: 100% / length($colors);

  @for $i from 1 through length($colors) {
    $pop: nth($colors,$i);
    $new: $pop ($width * ($i - 1)), $pop ($width * $i);
    $gradient: join($gradient, $new, comma);
  }

  @return linear-gradient($position, $gradient);
}

$mycolors: red orange yellow green blue indigo violet;

.rainbow { 
  @include background-image(rainbow(left, $colors));
}