Matt Grey Primer vs Stirland Mud

How close are they?

ΔE 37.9, loose: different enough that you should pick on purpose. Stirland Mud is noticeably darker, more saturated than Matt Grey Primer.

Can I use Stirland Mud instead of Matt Grey Primer?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Stirland Mud is noticeably darker, more saturated than Matt Grey Primer.

Can I use Matt Grey Primer instead of Stirland Mud?

The same difference applies the other way round: Matt Grey Primer is noticeably lighter, 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.