Looking at the sources, the description field supports HTML rendering (it is looking for and at the beginning and end of the text), so the following is supposed to work:
<field type="staticText" align="left" conditionid="con_jboss_installed">
<description><html><body><p style="color: red;">Please enter the Jboss directory name.</p></body></html></description>
</field>
This would require IzPack 5.