One of the things that we have to be very careful with when we talk about tooth color is not to imagine tooth colors to be like solid colors or solid paints. Tooth color is only one element of what we would call tooth appearance. So there's a lot more to tooth appearance than just color. Tooth appearance is optics, translucency, anatomy, surface, texture, the relation to its surroundings, position in the art. All of this is what contributes to tooth appearance. And color, in the terms of colorimetry, is just one aspect of that. But it doesn't describe the whole thing holistically.
It is quite interesting to realise that an A3 shade doesn't exist. Neither does any shade in our traditional shade guides. The reason it doesn't exist is because there is no ISO standard for it. So nobody ever said, we call an A3 an A3 if you take a tooth color sample with a thickness of a millimeter, and you measure its reflectance with this geometry, and if the values are this and that and then we call it an A3. No such standard exists.
Watch or listen to our the video podcast with Sasha Hein and A/Prof Max Guazzato discussing colour science and the e-LAB system.