Stuart contracts the high gloss finishes for USA SPECTORs to a company that specializes in spray-on baked finishes. The solid colors are also done by the same company. It's just a few miles down the road from SPECTOR in Woodstock.
It used to be that he didn't like doing solid finishes when he did them in-house because you have to clean the spray gun out and it took a lot of extra time.
In addition the NY Workshop is not horribly big. So he's never really had the space to build a proper finishing booth.
White was always a nightmare color because if there was any residual color from another finish in the gun or the nozzle it would ruin the finish. And gloss solid colors show every flaw. So dust, a small hair, anything.
The reason you don't see a lot of the solid color basses for the NS-Body USA or Euro Basses is because part of their appeal through the last two decades has been the beautiful exotic woods used in the construction.
When placing solid AAAA Quilt Maple wings on a bass, why would you want to cover them up?
The last variable is not a lot of people order solid-color USA or Euro models. Remember that until last year with the creation of the USA Stocking Dealers (who have to keep at least 3 USA Series basses or guitars in stock at all times), each USA Series bass or guitar was made to order. Someone one chose the finish and ordered it the way they wanted.
Gloss white (ala the Sting Bass) is the most common solid special order color. Ironically solid black is very rare. I have the only solid black NS-2 made in several years.
So it's just a matter of what someone wants. Regardless, the NS-2 will have AAAA Quilt Maple wings... so I think a lot of people who order one think "why cover something like that up?"
None of the USA models I order (I order one a year) will have solid finishes. Only the black one I have which was a special order that someone then decided not get at the last minute... mine will always be stained or natural finished wood.