when you add an Ominus banner to a shield it says "white shield" and the main color for it turns white instead of grey (I don't have a resource pack on. also in the image, it doesn't look that white because I have my mouse on it but it is)
MC-176907
Ominus banner on shield defaults to white
Resolved
Invalid
0
This isn't a bug. The Ominous Banner is built from a white banner, which is why its eyes have white instead of another color. As such, your shield is not a light grey or a grey shield, but a white shield, and the main color can't have changed to white because it was white all along. I understand your confusion, but I think the shield is white in order to create an illager face on a banner with a minimal amount of patterns applied.