The Blu Mediterraneo of my car is indeed a factory colour. But mine was the only such (UK, RHD) GTS produced in it.
The running costs for the full-fat GTS are the same, or lower, than the standard Sport GT. The GTS has the ZF auto box with paddle shift over-riders, whereas the pre-2009 Sport GT cars had the earlier Duo-Select manual box (which means a clutch change every X years).
Over and above the spec of a Sport GT, the GTS has lowered and fixed rate Bilstein sports dampers, sports seats, bigger brakes, bespoke tyres, carbon fibre interior parts, alcantara seat inserts (and gearknob, steering wheel, roof lining, etc), shadow chrome exterior kit (window surrounds, exhaust tips), colour coded door handles (and possibly some other things that currently slip my mind). It all adds up to provide a more engaging drive and a greater sense of exclusivity.
Of course, it comes at a cost - but only a modest one. My car is only a few £k over a similar age/mileage non-GTS QP.
If you are interested, PM me your email address and I'll send you full details/pictures. Though there's another thread on my car somewhere on the forum so you may see it there. It's not currently publicly advertised as I thought I'd wait until the spring and put it up for £22,995. But I'd happily move it on before hand for a couple of thousand less.