It is worded strangely but I believe they mean Rosewood Metallic is the base color with all other options as "Premium".Where/when did you hear this stated? Correct me if I'm wrong, but you're saying only Rosewood and two-tone paints don't cost extra? That would only leave Gun Metallic and Black Diamond Metallic costing extra which doesn't make sense. Gun Metallic, Brilliant Silver, and Black all come at no extra cost for all cars in their current line. For all of Nissan's current cars the TriCoat exterior paint colors all cost extra. Only seems logical that TriCoat would cost extra for the Z as well so I'm not sure what you heard is accurate. And I believe Ikazuchi Yellow is available for all Z trims, not just the Proto spec cars; seeing as a Base model Z was recently spotted in Ikazuchi Yellow.
This could vary in other markets.
I said Z Proto Spec will only be available in Ikazuchi Yellow. Not that this color is not exclusive to the Proto.