ΔE 21.6, loose: different enough that you should pick on purpose. Mythic Turquoise is noticeably darker, more saturated than Marine Mist.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Mythic Turquoise is noticeably darker, more saturated than Marine Mist.
The same difference applies the other way round: Marine 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.
See Marine Mist on the colour wheel See Mythic Turquoise on the colour wheel
All matches: Marine Mist equivalents · Mythic Turquoise equivalents