ΔE 11.7, fair: the same colour family, but not a straight swap. Sepia Vetus is cooler than Seraphim Sepia. They are different kinds of paint: Citadel Seraphim Sepia is a wash/shade and Green Stuff World Sepia Vetus 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). Sepia Vetus is cooler than Seraphim Sepia. They are different kinds of paint.
The same difference applies the other way round: Seraphim Sepia is warmer.
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 Seraphim Sepia on the colour wheel See Sepia Vetus on the colour wheel
All matches: Seraphim Sepia equivalents · Sepia Vetus equivalents