Generated by SassMeister.com.

SassMeister-input.scss

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

@mixin button($background-color) {
  color: if( lightness($background-color) > 50%, black, white); // do magic here
  background: $background-color;
}

.button-green {
  @include button(#c69);
}

.button-gray {
  @include button(#333);
}

SassMeister-output.css

.button-green {
  color: black;
  background: #c69;
}

.button-gray {
  color: white;
  background: #333;
}