ΔE 5.5, very close: most people won’t see a difference side by side. Sepia Vetus is a little lighter than Mid Mustard Khaki. They are different kinds of paint: Army Painter Mid Mustard Khaki 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.
Yes, for most jobs. Very close match (ΔE 6). Sepia Vetus is a little lighter than Mid Mustard Khaki. They are different kinds of paint.
The same difference applies the other way round: Mid Mustard Khaki is a little darker.
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 Mid Mustard Khaki on the colour wheel See Sepia Vetus on the colour wheel
All matches: Mid Mustard Khaki equivalents · Sepia Vetus equivalents