FIAT boffins would know the reasons! But, in general the solid paints are less expensive than metallic ones. Also, the process of painting the "premium" colors and the finishing may be different ; contributing to difference in costs. That said, i find no difference in premium or "ordinary". It all boils down to how well we care for the paint. I've seen a FTA Linea ( premium shade) look much drab and ordinary than BNW. Cheers.