Yes, I presume that's how it works, but each colour has it's own supply - which makes me realise that there are 4 cores to consider here - so if I go for 2.5mm x 4, there would be only about 1.5% VD. Doh!
...although I'm not sure about this now, as although the current to each colour goes down a separate core, the return current will be shared on the 4th core. I suppose what I need is 4 core cable where one of the cores is nice and fat (which is probably how one might describe the chance of such a cable existing!).