Caribbean Ocean vs Graphite Green

How close are they?

ΔE 43.8, loose: different enough that you should pick on purpose. Graphite Green is noticeably darker, much more muted than Caribbean Ocean.

Can I use Graphite Green instead of Caribbean Ocean?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 44). Graphite Green is noticeably darker, much more muted than Caribbean Ocean.

Can I use Caribbean Ocean instead of Graphite Green?

The same difference applies the other way round: Caribbean Ocean is noticeably lighter, 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.