ΔE 8.7, close: similar, with a visible difference side by side. Sepia Vetus is a little darker, cooler than Karak Stone. They are different kinds of paint: Citadel Karak Stone is an opaque base/layer paint 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.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Sepia Vetus is a little darker, cooler than Karak Stone. They are different kinds of paint.
The same difference applies the other way round: Karak Stone is a little lighter, 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 Karak Stone on the colour wheel See Sepia Vetus on the colour wheel
All matches: Karak Stone equivalents · Sepia Vetus equivalents