ΔE 23.2, loose: different enough that you should pick on purpose. Primer Black is noticeably darker, much more muted than Stirland Mud.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Primer Black is noticeably darker, much more muted than Stirland Mud.
The same difference applies the other way round: Stirland Mud is noticeably lighter, much more saturated.
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 Primer Black on the colour wheel
All matches: Stirland Mud equivalents · Primer Black equivalents