ΔE 10.1, fair: the same colour family, but not a straight swap. Nuln Oil is a little lighter, more muted than Sepia INK. They are different kinds of paint: AK Interactive Sepia INK is an ink, glaze or transparent paint and Citadel Nuln Oil is a wash/shade, 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 10). Nuln Oil is a little lighter, more muted than Sepia INK. They are different kinds of paint.
The same difference applies the other way round: Sepia INK is a little darker, more saturated.
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 Sepia INK on the colour wheel See Nuln Oil on the colour wheel
All matches: Sepia INK equivalents · Nuln Oil equivalents