Maybe custom paint jobs cost more to repair, or perhaps the insurance companies figure that is you can afford custom paint then you can afford a higher premium. They go on about their algorithms calculating risk of the rider, bike, location and usage etc that is all blended with historical statistics to formulate the insurance premiums. Well I think the algorithm simply works out how much they can fleece their punters for without them going to another company.
I think they do just take any opportunity to class a modification or change as an excuse to increase the premiums. The new paint job isn't a custom one, just straight Triumph Competition Green. I also changed the bars for lower ones as I find high bars give me back ache, that also counts as a mod whereas it is the equivalent of adjusting your car seat. It is all just a rip off, not that that is news to anyone!