No, it’s valid for any observer. Between two colours there’s a third no-one thought of before.
Well, since human colour vision varies significantly depending on individual physiology, lighting conditions, surrounding context, acquaintance and other factors, it’s not even unique to have unique colour experiences.
We can, of course, assume some average and call it the human gamut. In the Technology & Science category you can find a topic titled Enhanced Color Vision. There you can find a link to an article about people who can see a colour outside the human gamut with the help of lasers.