Bootstrap/scss/helpers/_colored-links.scss

@each $color, $value in $theme-colors {
  .link-#{$color} {
    color: $value if($enable-important-utilities, !important, null);

    @if $link-shade-percentage != 0 {
      &:hover,
      &:focus {
        color: if(color-contrast($value) == $color-contrast-light, shade-color($value, $link-shade-percentage), tint-color($value, $link-shade-percentage)) if($enable-important-utilities, !important, null);
      }
    }
  }
}