You could use the Color Changer tool in PSP, which I always find a bit of a fiddle to use, or MV's Plugins>Color Replacer, I think that is a much better tool, it's easier and more accurate.
It's free and you can get it here:
http://vicanek.de/plugins/colorreplacer.htm

Place it in your Plugins folder and point PSP to it in File>Preferences>File Locations.
:-)
Grenou