$shade: mix(#000000, #bbb, 10%);
$tint: mix(#ffffff, #bbb, 10%);
// $nexted: mix(#000000, tint($black, 24%), 10%);

@mixin gradient-vertical($startColor: #555, $endColor: #333){

}
@include gradient-vertical(mix(#000000, $yellow, 80%), mix(#ffffff, $yellow, 70%));