Originally Posted by
Yuriane
I think that scene actually tells us a lot about aethersight in general. They describe it how we see it, then she says she's seeing the same thing. This gives us two pieces of info.
1 - The illusion is solid aether, aether itself can be molded to shape and colour and mass, this is likely true for primals as well also explaining why many primals only have one major colour given they're comprised of mostly one element of aether.
2 - Aethersight functions simply by only being able to see aether. There's more than likely a range to which that aether can be seen and stronger manifestations of aether are more easily visible. Her being able to see the illusion the same way we do shows that she's unable to see things conventionally which is why it's so significant she has a conventional sight over the illusion. We can also infer from not just this but other instances that her aethersight is like a multicoloured variant of the aetherscopes, as the aetherscopes make no distinction between certain types of aether while she's able to tell each element individually. This is likely the case for the paint.
It's important to remember that aether isn't just vaugely around but everything has aether in it. The land needs aether to grow, develop, and die. A small part of the Eden raids show us it's near impossible to light a fire with no ambient aether present, even through nonmagical means. When something is "aether-infused" it's could be better described as "aether saturated" and thus the aether within is usable. With practice, which Y'shtola has more than enough opportunity to have, one can possibly see such thinly spread aether.