It has come to my attention that The "Gimp Rainbow Brushes" were not the correct file type.
I have replaced this brush set with the correct file type (ie. .gbr which is the Gimp brush file extension)...
These new rainbow brushes are better then the original one, in that the resolution in much higher and the brush size has also been increased.
Sorry for any inconvenience this my have caused.
Gimp Rainbow Brushes
[link]