Deep Ocean Blue vs Slate Grey

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Slate Grey is a little lighter, more muted than Deep Ocean Blue.

Can I use Slate Grey instead of Deep Ocean Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Slate Grey is a little lighter, more muted than Deep Ocean Blue.

Can I use Deep Ocean Blue instead of Slate Grey?

The same difference applies the other way round: Deep Ocean Blue is a little 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.