RGB

rgb($red, $green, $blue) Creates a Color from red, green, and blue values.
rgba($red, $green, $blue, $alpha) Creates a Color from red, green, blue, and alpha values.
red($color) Gets the red component of a color.
green($color) Gets the green component of a color.
blue($color) Gets the blue component of a color.
mix($color1, $color2, [$weight]) Mixes two colors together.
For mix(), the weight specifies the amount of the first color that should be included in the returned color.
$a : #369;
$b : rgb(blue($a), green($a), red($a));

div {
   background: $b;
   color: mix(#000, #999, 30%);
}

div {
  background: #996633;
  color: #6b6b6b;
}