Black Tone vs Mortarion Grime

How close are they?

ΔE 34.9, loose: different enough that you should pick on purpose. Mortarion Grime is noticeably lighter, much more saturated than Black Tone.

Can I use Mortarion Grime instead of Black Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 35). Mortarion Grime is noticeably lighter, much more saturated than Black Tone.

Can I use Black Tone instead of Mortarion Grime?

The same difference applies the other way round: Black Tone is noticeably darker, 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.