A Sass mixin for quickly setting link states – test it on http://sassmeister.com/ (see more examples in the comments)

_link_states.scss

/*
  All arguments *may* be space separated lists:
    @include link-states(<properties>, <normal values>[, <state values>]
      [, <:active values>][, <:focus values>]);

  Example:
    a {
      @include link-states(color, red, blue);
    }
  
  Output:
    a {
      color: red;
    }
    a:hover, a:focus, a:active {
      color: blue;
    }
    
  If <normal value> is a color, then positive percentages in state values
  will lighten the color and negative percentages will darken it
  
  Values cascade. Use null to prevent outputting a value
*/
 
@mixin link-states(
  $properties,
  $values,
  $states-values: $values,
  $active-values: $states-values,
  $focus-values: $states-values) {

  @if length($properties) > 1 {
    @for $i from 2 to length($properties) + 1 {
      @if length($values) < $i { $values: append($values, nth($values, length($values))); }
      @if length($states-values) < $i { $states-values: append($states-values, nth($states-values, length($states-values))); }
      @if length($active-values) < $i { $active-values: append($active-values, nth($active-values, length($active-values))); }
      @if length($focus-values) < $i { $focus-values: append($focus-values, nth($focus-values, length($focus-values))); }
    }
  }

  $prop-val-list: zip(
    $properties,
    $values,
    $states-values,
    $active-values,
    $focus-values
  );

  $states-sel: unquote('&:hover');
  $active-sel: unquote('&:active');
  $focus-sel: unquote('&:focus');

  @if $active-values == $states-values {
    $states-sel: append($states-sel, $active-sel, comma);
    $active-sel: null;
  }
  @if $focus-values == $states-values {
    $states-sel: append($states-sel, $focus-sel, comma);
    $focus-sel: null;
  }

  @each $prop-val in $prop-val-list {
    $property: nth($prop-val, 1);
    $value: nth($prop-val, 2);
    
    #{$property}: $value;
  }

  #{$states-sel} {
    @each $prop-val in $prop-val-list {
      $property: nth($prop-val, 1);
      $value: nth($prop-val, 2);
      $states-value: nth($prop-val, 3);
    
      @if type-of($value) == 'color' {
        @if type-of($states-value) == 'number' and unit($states-value) == '%' {
          @if $states-value < 0% { $states-value: darken($value, $states-value * -1); }
          @else { $states-value: lighten($value, $states-value); }
        }
      }
      
      #{$property}: $states-value;
    }
  }

  @if $active-sel {
    #{$active-sel} {
      @each $prop-val in $prop-val-list {
        $property: nth($prop-val, 1);
        $value: nth($prop-val, 2);
        $active-value: nth($prop-val, 4);
      
        @if type-of($value) == 'color' {
          @if type-of($active-value) == 'number' and unit($active-value) == '%' {
            @if $active-value < 0% { $active-value: darken($value, $active-value * -1); }
            @else { $active-value: lighten($value, $active-value); }
          }
        }
        
        #{$property}: $active-value;
      }
    }
  }

  @if $focus-sel {
    #{$focus-sel} {
      @each $prop-val in $prop-val-list {
        $property: nth($prop-val, 1);
        $value: nth($prop-val, 2);
        $focus-value: nth($prop-val, 5);
      
        @if type-of($value) == 'color' {
          @if type-of($focus-value) == 'number' and unit($focus-value) == '%' {
            @if $focus-value < 0% { $focus-value: darken($value, $focus-value * -1); }
            @else { $focus-value: lighten($value, $focus-value); }
          }
        }
      
        #{$property}: $focus-value;
      }
    }
  }
}

It is easy to create more convenient mixins using rest syntax ("..."):

@mixin link-color($params...) {
  @include link-states(color, $params...);
}

/*
  Example:
    a {
      @include link-color(null, blue, brown);
    }

  Output:
    a:hover, a:focus {
      color: blue;
    }
    a:active {
      color: brown;
    }
*/