ΔE 11.8, fair: the same colour family, but not a straight swap. Ink Sepia is a little lighter, more saturated than Abaddon Black. They are different kinds of paint: Citadel Abaddon Black is an opaque base/layer paint and Vallejo Ink Sepia 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). Ink Sepia is a little lighter, more saturated than Abaddon Black. They are different kinds of paint.
The same difference applies the other way round: Abaddon Black is a little 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 Ink Sepia on the colour wheel
All matches: Abaddon Black equivalents · Ink Sepia equivalents