ΔE 48.5, loose: different enough that you should pick on purpose. Primer Black Brown is noticeably darker than Matt Grey Primer.
Can I use Primer Black Brown instead of Matt Grey Primer?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 49). Primer Black Brown is noticeably darker than Matt Grey Primer.
Can I use Matt Grey Primer instead of Primer Black Brown?
The same difference applies the other way round: Matt Grey Primer is noticeably lighter.
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.