_color.scss

@import "compass/css3";
@function relativeLuminance($color) {
  @return 0.2126 * red($color)/255.0 + 
          0.7152 * green($color)/255.0 +
          0.0722 * blue($color)/255.0;
}

@function contrast($color1,$color2) {
  $luminance1: relativeLuminance($color1);
  $luminance2: relativeLuminance($color2);
  $ratio: ($luminance1 + 0.05)/($luminance2 + 0.05);
  @if ($ratio < 1) {
    @return 1/$ratio;
  } @else {
    @return $ratio;
  }
}

// Add percentage of white to a color
@function tint($color, $percent){
  @return mix(white, $color, $percent);
}
 
// Add percentage of black to a color
@function shade($color, $percent){
  @return mix(black, $color, $percent);
}

@function guessGoodColor($color) {
  $newColor: $color;
  $oldContrast: 0.0;
  
  @if relativeLuminance($color) > 0.5 {
    @for $i from 0 to 100 {
      $paletteColor: shade($color, percentage($i/100));
      
      @if (contrast($color, $paletteColor) > 2.0 
        and contrast($color, $paletteColor) > $oldContrast
        and contrast($color, $paletteColor) < 5.5) {
        $newColor: $paletteColor;
        $oldContrast: contrast($color, $paletteColor);
      }
    }  
  } @else {
    @for $i from 0 to 100 {
      $paletteColor: tint($color, percentage($i/100));
    
      @if (contrast($color, $paletteColor) > 2.0 
        and contrast($color, $paletteColor) > $oldContrast
        and contrast($color, $paletteColor) < 5.5) {
        $newColor: $paletteColor;
        $oldContrast: contrast($color, $paletteColor);
      }
    }
  }

    
  @return $newColor;
}

@mixin goodCombi($bgColor){
  $text-color: guessGoodColor($bgColor);
  background-color: $bgColor;
  color: $text-color;
}

body {
  @include goodCombi(red);
}