Uses Compass' contrast-color function (http://compass-style.org/reference/compass/utilities/color/contrast/).

SassMeister-input.scss

// ----
// Sass (v3.4.7)
// Compass (v1.0.1)
// ----

@import "compass";

$color: #C6538C;

.box {
  background-color: $color;
  color: contrast-color($color, purple, yellow);
  padding: .5rem;
  max-width: 25rem;
}

SassMeister-output.css

.box {
  background-color: #C6538C;
  color: yellow;
  padding: .5rem;
  max-width: 25rem;
}