Toxic Mist vs Light Turquois (Phthalo)

How close are they?

ΔE 19.0, loose: different enough that you should pick on purpose. Light Turquois (Phthalo) is noticeably darker, warmer than Toxic Mist.

Can I use Light Turquois (Phthalo) instead of Toxic Mist?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Light Turquois (Phthalo) is noticeably darker, warmer than Toxic Mist.

Can I use Toxic Mist instead of Light Turquois (Phthalo)?

The same difference applies the other way round: Toxic Mist is noticeably lighter, 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.