I don't just want to change the color of the text as I'm doing the PHP scripting, I want the outputted text do be a different color.
If I understand correctly, you're worried about changing the font color of the PHP code itself? PHP code is just plaintext on the server... if you're echo'ing text, simply echo the HTML:
echo "This is <font color=\"blue\">blue</font>!";
NOTE: In your above example, to output the text in green, you would do:
echo '<font color="green">' . HEADING_TITLE . '</font><br>';