Generated by SassMeister.com.

SassMeister-input-HTML.html

<span class="inset-one">ello</span>
<span class="inset-two">arry</span>
<span class="inset-3">potter</span>

SassMeister-input.scss

// ----
// libsass (v0.7.0)
// ----

@mixin text-inset($color, $opacity: 0.5) {
 
  $shadow-color: desaturate(invert($color), 100%);
 
  @mixin lightness {
    content: 'lightness' ;
    font-size: .5em;
    display:block;
  }
  
  @if lightness($color) < 50% {
  text-shadow: 1px 1px 1px rgba($color, $opacity);
  &:after {
    @include lightness;
  }
} @else {
   text-shadow: 1px 1px 1px rgba($color, $opacity);
  &:after {
    @include lightness;
}
}
color: $color;
}

body {
  background-color: #999;
  font-size: 6em;
  padding: .1em;
}

%inset {
  font-weight:bold;
  text-transform: uppercase;
  display: block;
}

.inset-one {
  @extend %inset;
  @include text-inset(pink)
}

.inset-two {
  @extend %inset;
  @include text-inset(green)
}

.inset-3 {  
  @extend %inset;
  @include desaturate(orange)
}

SassMeister-output.css

source string:49: error: no mixin named desaturate
Backtrace:
	source string:49

SassMeister-rendered.html

<span class="inset-one">ello</span>
<span class="inset-two">arry</span>
<span class="inset-3">potter</span>