Texturing a reskin

I'm considering doing a reskin of some BR Diesels, and one I'm thinking about doing is a good model but the skin of it basically uses a block of colour for the main body side. I'd like to reskin it to give it a slightly more realistic looking body side.

Can anyone give me any tips? I use GIMP but I'm no expert. Also, any tips on adding weathering would be lovely too.

Cheers

You'll get more useful tips if you provide the KUID of the model you contemplate adding a texture to. Be warned, though, that the skins of some models are made in a way that makes it impossible, or at least very difficult to add a texture by any than the original creator.
ns

I was thinking of having a play with Snowsignal's class 56 in BR Blue...
Alternatively one of the AC Electrics - 86 or 87.