Stirland Mud vs Primer Black Brown

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Primer Black Brown is noticeably darker, more muted than Stirland Mud.

Can I use Primer Black Brown instead of Stirland Mud?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Primer Black Brown is noticeably darker, more muted than Stirland Mud.

Can I use Stirland Mud instead of Primer Black Brown?

The same difference applies the other way round: Stirland Mud is noticeably lighter, 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.