SCSSの繰り返しと:nth-child(n)の組み合わせ

8色$colorsを順番に回す

div {
  $length: length($colors);
  @each $color in $colors {
    $i: index($colors, $color);
    &:nth-child(#{$length + "n+" + $i}) {
      border-color: $color;
      
      &::before {
        border-color: $color;
      }
    }
  }
}