ΔE 71.7, loose: different enough that you should pick on purpose. Stirland Mud is noticeably darker, much more saturated than Matt White Primer.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 72). Stirland Mud is noticeably darker, much more saturated than Matt White Primer.
The same difference applies the other way round: Matt White Primer is noticeably lighter, much more muted.
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 Matt White Primer on the colour wheel See Stirland Mud on the colour wheel
All matches: Matt White Primer equivalents · Stirland Mud equivalents