This is my first experimentation with GIMP Plugin Architecture ..
Download this Plugin C file Plugin Source
Download the Windows plugin binary in Zip format for GIMP 2.2 bw2rgb.zip
Expand the above zip file
Copy this bw2rgb.exe file to C:\Documents and Settings\Administrator\.gimp-2.2\plug-ins folder on your windows box (or) copy this to your plugin folder for the GIMP Installation in windows
Restart GIMP
Open a black and white RGB image file.
Choose menu option Filters -> Colors -> bw2color -> Black & White to color