A filter format for applying greentext correctly to Drupal text. The snippet will provide any filter format with the ability to change any line starting with '>' into a greentext line. >Implying anyone will use it

greentext.module

<?php

function greentext_filter_info() {
  $filters['greentext'] = array(
    'title' => t('Implying greentext'),
    'description' => t('Allows you to alter any lines input starting with \'>\' to appear as greentext.'),
    'process callback' => '_greentext_greentext',
    'tips callback' => '_greentext_greentext_tips',
  );
  return $filters;
}

function _greentext_greentext($text, $filter) {
  $lines = explode("\n", $text);
  foreach ($lines as $index => $line) {
    if (preg_match('/^>/', strip_tags($line), $matches)) {
      $lines[$index] = theme('greentext', array('text' => $line));
    }
  }
  $text = implode("\n", $lines);

  return $text;
}

function _greentext_greentext_tips($filter, $format, $long = FALSE) {
  $output = t('Lines beginning with \'>\' will automatically be replaced with greentext');
  return $output;
}

function greentext_theme($existing, $type, $theme, $path) {
  return array(
    'greentext' => array(
      'template' => 'greentext',
      'variables' => array('text' => NULL),
    ),
  );
}

greentext.tpl.php

<?php

// Put this in a CSS file and add class.
print '<span style="color:#789922">' . $text . '</span>';

This can also be done with PHP's DOMDocument library. Code:
https://www.256kilobytes.com/content/show/4285/how-do-i-make-create-greentext-4chan-style-using-the-php-domdocument-library