think you have it covered paul
easiest way to tell is look under the bonnet,homemade ones normally have all sorts of bits/pieces missing,clips and brackets etc, use a mismatch of engine parts from various models etc,but if you dont know what to look for or are new to the scene then it can be a nightmare! only problem is proving who actually built them in the first place, was it some rallying company or someone in thier shed? ive seen some very poorly built examples,as far as i know the factory versions were only available in white as you say