Dark Blue Tone vs Atramentum Black

How close are they?

ΔE 24.6, loose: different enough that you should pick on purpose. Atramentum Black is noticeably lighter, much more muted than Dark Blue Tone.

Can I use Atramentum Black instead of Dark Blue Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Atramentum Black is noticeably lighter, much more muted than Dark Blue Tone.

Can I use Dark Blue Tone instead of Atramentum Black?

The same difference applies the other way round: Dark Blue Tone is noticeably darker, much 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.