Dark Blue Black vs Viking Grey

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Viking Grey is noticeably lighter, more muted, cooler than Dark Blue Black.

Can I use Viking Grey instead of Dark Blue Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Viking Grey is noticeably lighter, more muted, cooler than Dark Blue Black.

Can I use Dark Blue Black instead of Viking Grey?

The same difference applies the other way round: Dark Blue Black is noticeably darker, more saturated, warmer.

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.