Toxic Mist vs Mythic Turquoise

How close are they?

ΔE 21.6, loose: different enough that you should pick on purpose. Mythic Turquoise is noticeably darker, more saturated than Toxic Mist.

Can I use Mythic Turquoise instead of Toxic Mist?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Mythic Turquoise is noticeably darker, more saturated than Toxic Mist.

Can I use Toxic Mist instead of Mythic Turquoise?

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

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.