ΔE 11.8, fair: the same colour family, but not a straight swap. Opulentus Black is noticeably lighter, more saturated than Abaddon Black. They are different kinds of paint: Citadel Abaddon Black is an opaque base/layer paint and Green Stuff World Opulentus Black is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Opulentus Black is noticeably lighter, more saturated than Abaddon Black. They are different kinds of paint.
The same difference applies the other way round: Abaddon Black is noticeably darker, more muted.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Abaddon Black on the colour wheel See Opulentus Black on the colour wheel
All matches: Abaddon Black equivalents · Opulentus Black equivalents