Deep Ocean Blue vs Darkness Grey

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Darkness Grey is noticeably lighter, more muted than Deep Ocean Blue.

Can I use Darkness Grey instead of Deep Ocean Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Darkness Grey is noticeably lighter, more muted than Deep Ocean Blue.

Can I use Deep Ocean Blue instead of Darkness Grey?

The same difference applies the other way round: Deep Ocean Blue is noticeably darker, 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.