ΔE 11.7, fair: the same colour family, but not a straight swap. Transparent Sepia is more saturated, warmer than Brownish Decay. They are different kinds of paint: Army Painter Brownish Decay is a one-coat contrast-style paint and Green Stuff World Transparent 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). Transparent Sepia is more saturated, warmer than Brownish Decay. They are different kinds of paint.
The same difference applies the other way round: Brownish Decay is more muted, cooler.
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 Brownish Decay on the colour wheel See Transparent Sepia on the colour wheel
All matches: Brownish Decay equivalents · Transparent Sepia equivalents