I think people need to know what colour the coach is so they can see it (and flag it down if at a roadside stop) but I don't think it's crucial whether it's pink, white or green. I do agree Flix's look is modern and attractive, but I don't think people would choose one over the other on that basis - because almost everyone willing to pay more uses another mode*, it just comes down to price for most.
* Scotland may be a bit different due to its poorly developed rail network, but if you've got a coach going from Manchester to London for £20 and a WMT train (with a change at Crewe) going for £25, the coach takes 5 hours and the train takes 3-3.5 depending on connections, almost everyone will pay that extra fiver unless on a *really* tight budget. Of course if GBR make a mess of that fare differential, it's possible coach's market could grow.