If you want certain words or phrases to appear in a different colour but don't want to get into the business of writing all your posts using full HTML code, then you will need to change the default styling for one of the few tags that Nabble does allow you to use without checking the "Message is in HTML Format" box.
The tags available for this treatment are: Bold, Italic and the headings H2-H6. While a few other tags may work, it would be unwise to rely on them as Nabble does not document their support.
To make a single instance text tagged as bold appear in red:
To generate red text you just add the code - style="font-weight: normal; color:red;" - to the opening "b" tag like this: