Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.4)
// Compass (v1.0.0.alpha.18)
// ----

@function set-color($color) {
    @if (lightness($color) > 40) {
      @return #000;
    }
    @else {
      @return #FFF;
    }
}

div {
    background-color: black; // Or whatever else
    p {
      color: set-color(black);
    }
}

SassMeister-output.css

div {
  background-color: black;
}
div p {
  color: white;
}