Marine Mist vs Light Phthalo Green

How close are they?

ΔE 16.4, loose: different enough that you should pick on purpose. Light Phthalo Green is noticeably lighter, more saturated, warmer than Marine Mist.

Can I use Light Phthalo Green instead of Marine Mist?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Light Phthalo Green is noticeably lighter, more saturated, warmer than Marine Mist.

Can I use Marine Mist instead of Light Phthalo Green?

The same difference applies the other way round: Marine Mist is noticeably darker, more muted, cooler.

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.