@function color($key) {
  @return map-get($colors, $key);
}

@function bc($key) {
  @return map-get($bc, $key);
}

// http://www.smashingmagazine.com/2014/06/12/sassy-z-index-management-for-complex-layouts/
@function z($element) {
  @return index($elements, $element);
}

@function tint($color, $amount) {
  @return scale-color($color, $lightness: $amount);
}

@function shade($color, $amount) {
  @return scale-color($color, $lightness: -$amount);
}

@function color($key) {
  @return map-get($colors, $key);
}

@function bc($key) {
  @return map-get($bc, $key);
}

// http://www.smashingmagazine.com/2014/06/12/sassy-z-index-management-for-complex-layouts/
@function z($element) {
  @return index($elements, $element);
}

// https://css-tricks.com/snippets/sass/tint-shade-functions/
@function tint($color, $amount) {
  @return scale-color($color, $lightness: $amount);
}
@function shade($color, $amount) {
  @return scale-color($color, $lightness: -$amount);
}

Add reference for the tint/shade functions
Add reference for the tint/shade functions
