Color-treated gold South Sea are pretty common and easy to spot at the shows. I've seen them here in the US and at the shows in Hong Kong. The treatments are disclosed as far as I've seen and the pearls are a dark gold but very inexpensive. Dark gold South Sea pearls are extremely expensive.
It's difficult to discern a natural-color GSSP from a treated pearl without some experience. A couple of telltale signs would be the very small appearance of what look like Newton rings, visible conchiolin that has changed colors usually appearing as a sort of reddish tone and dye concentrations in blemishes.
To give one an idea of the difference in value, you could buy a dark dyed strand for a thousand dollars. You could spend $10,000 on the same strand with natural colors. In larger sizes, the spread would increase.