Opacity

alpha($color)
opacity($color)
Gets the alpha component (opacity) of a color.
rgba($color, $alpha) Changes the alpha component for a color.
opacify($color, $amount)
fade-in($color, $amount)
Makes a color more opaque.
transparentize($color, $amount)
fade-out($color, $amount)
Makes a color more transparent.
The opacity of a color can be controlled explicitly.
$a: #abc;
$b: opacify(rgba($a,0.3),0.1);

div {
   background: $b;
}

div {
  background: rgba(170, 187, 204, 0.4);
}