A strange subject but it is relevant

I'm still going though my mod, its progressing quite well but I am a little short of decent ship models for the Cardassians (and some of the others tbh).
So I have found a nice model (Dr_Breen's Ranol class from his ship pack) but its colours/texture is not so great. So I would like to be able to alter these colours to make it fit in with the other Cardassian models (almost all taken from the UDM ship pack).
I have tried the indexed pallet swap trick mentioned in the reducing paletts topic, but I only have the GIMP as my graphics package (a bit more limited than photoshop) and the results look worse than the original.
So, as per the subject, my art and design skills are some what limited but I am willing to try out some different things to get this to work. Can anyone suggest a good (and free, free being the priority here

I am not worried at the moment with changing the actual model, just its texture, so any help with this will be much appreciated

Regards,
C.