ΔE 12.0, fair: the same colour family, but not a straight swap. Mahogany Ink is a little lighter, cooler than Stirland Mud. They are different kinds of paint: Citadel Stirland Mud is a technical paint or medium and Vallejo Mahogany Ink 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). Mahogany Ink is a little lighter, cooler than Stirland Mud. They are different kinds of paint.
The same difference applies the other way round: Stirland Mud is a little darker, 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 Stirland Mud on the colour wheel See Mahogany Ink on the colour wheel
All matches: Stirland Mud equivalents · Mahogany Ink equivalents