Bootstrap/scss/helpers/_colored-links.scss

@each $color, $value in $theme-colors {
  .link-#{$color} {
    color: $value;

    @if $emphasized-link-hover-darken-percentage != 0 {
      &:hover,
      &:focus {
        color: if(color-contrast($value) == $color-contrast-light, darken($value, $emphasized-link-hover-darken-percentage), lighten($value, $emphasized-link-hover-darken-percentage));
      }
    }
  }
}