No, Umbral is stasis, which is Light.
Astral is creation and Darkness.
This is stated... in the MSQ, in Il Mheg...
I don't think you were paying close enough attention.
There's a reason why Shadowbringers has turned our understanding of Astral/Umbral on its head, why we now realise that the naming of Astral/Umbral era's is a misnomer.