How I'd implement https://gist.github.com/2190496 in Sass

color_shifted.scss

@function color-shift($color, $amount: 10%) {
  @return if(lightness($color) > 50%,
             darken($color, $amount),
             lighten($color, $amount))
}

$my-bg-color: #333;
$my-fg-color: #c00;

.shifted {
  background-color: color-shift($my-bg-color);
  color: color-shift($my-fg-color);
}