sass-hash-mixin.sass

@mixin category-colors($after: false)
  @each $category in $categories
    @if $after == true
      .cat-#{nth($category, 1)}:after
        background-color: nth($category, 2)
    @else
      .cat-#{nth($category, 1)}
        background-color: nth($category, 2)

stylesheet.sass

// Categories & Category Colors
// this is a comma-separated list of space-separated lists!
$categories: site teal, youth orange, poetry plum, fiction salmon, creativewriting steelblue

// The placeholder selector we'll be @extending -- placeholder selectors require Sass 3.2
%category-colors
  @include category-colors

.category-banners
  li
    // @extend the color classes into the list items
    @extend %category-colors

stylesheet_output.css

/* Note that the placeholder class isn't output in the CSS */

.category-banners li .cat-site {
  background-color: teal; 
}
.category-banners li .cat-youth {
  background-color: orange; 
}
.category-banners li .cat-poetry {
  background-color: plum; 
}
.category-banners li .cat-fiction {
  background-color: salmon; 
}
.category-banners li .cat-creativewriting {
  background-color: steelblue; 
}