Not true. Human tetrachromats have an extra kind of receptor somewhere between the blue and red receptors' sensitivity. This doesn't help with colors like violet that are outside of that range.
Also, purple (a non-spectral color) is easily distinguished from violet (a spectral color) if you see them side-by-side.
There exists a shade of purple that is indistinguishable from violet because it triggers the cones of the eyes at the same level that violet does.
You can buy paint called "violet". This isn't the spectral violet, it's a shade of purple that looks very similar to spectral violet.
Tetrochromats can distinguish between that purple shade and real violet. But if you mixed the paint using 4 tints rather than 3 you could fool them too.
edit: You may be confusing tetrachromacy with people who don't have a lens and can therefore perceive ultraviolet light that's normally filtered out. These folks can see shades of violet where other people don't because the blue receptors are being stimulated by the ultraviolet light.
