ΔE 16.3, loose: different enough that you should pick on purpose. Phthalo Turquoise Light is noticeably darker, more saturated, cooler than Fenrisian Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Phthalo Turquoise Light is noticeably darker, more saturated, cooler than Fenrisian Grey.
The same difference applies the other way round: Fenrisian Grey is noticeably lighter, more muted, warmer.
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 Fenrisian Grey on the colour wheel See Phthalo Turquoise Light on the colour wheel
All matches: Fenrisian Grey equivalents · Phthalo Turquoise Light equivalents