Sass function to check contrast ratio & conditionally change text color

SassMeister-input-HTML.html

<button>Submit</button>

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.3)
// Compass (v1.0.0.alpha.18)
// ----

/* Awesome contrast ratio function
* via https://gist.github.com/voxpelli/6304812
**/
@function color_luminance($color) {
  // Adapted from: https://github.com/LeaVerou/contrast-ratio/blob/gh-pages/color.js
  // Formula: http://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
  $rgba: red($color), green($color), blue($color);
  $rgba2: ();
 
  @for $i from 1 through 3 {
    $rgb: nth($rgba, $i);
    $rgb: $rgb / 255;
 
    $rgb: if($rgb < .03928, $rgb / 12.92, pow(($rgb + .055) / 1.055, 2.4));
 
    $rgba2: append($rgba2, $rgb);
  }
 
  @return .2126 * nth($rgba2, 1) + .7152 * nth($rgba2, 2) + 0.0722 * nth($rgba2, 3);
}

@function color_contrast($color1, $color2) {
  // Adapted from: https://github.com/LeaVerou/contrast-ratio/blob/gh-pages/color.js
  // Formula: http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
  $luminance1: color_luminance($color1) + .05;
  $luminance2: color_luminance($color2) + .05;
  $ratio: $luminance1 / $luminance2;
 
  @if $luminance2 > $luminance1 {
    $ratio: 1 / $ratio;
  }
 
  $ratio: round($ratio * 10) / 10;
 
  @return $ratio;
}

$button-blue: #3879B3;

@mixin button($color: $button-blue){
  
	display: inline-block;
	vertical-align: top;
	padding: 2px 10px 3px;
	-webkit-appearance: none;
	cursor: pointer;
	border-style: solid;
	border-color: darken($color,15%);
	border-width: 1px 1px 2px;
	background-color: $color;
	border-radius: 3px;
	// Check contrast ratio of white to background color
	@if color_contrast($color, #FFF) < 4.5{
		color: #111;
		text-shadow: none;
	}
	@else{
		color: #FFF;
		text-shadow: 0 1px #000;
	}
}

button,
input[type="button"],
input[type="reset"],
input[type="submit"]{
	@include button;
}

SassMeister-output.css

/* Awesome contrast ratio function
* via https://gist.github.com/voxpelli/6304812
**/
button,
input[type="button"],
input[type="reset"],
input[type="submit"] {
  display: inline-block;
  vertical-align: top;
  padding: 2px 10px 3px;
  -webkit-appearance: none;
  cursor: pointer;
  border-style: solid;
  border-color: #265279;
  border-width: 1px 1px 2px;
  background-color: #3879b3;
  border-radius: 3px;
  color: #FFF;
  text-shadow: 0 1px #000;
}