Arctic Gem vs Phthalo Turquoise Light

How close are they?

ΔE 23.3, loose: different enough that you should pick on purpose. Phthalo Turquoise Light is much more muted, cooler than Arctic Gem.

Can I use Phthalo Turquoise Light instead of Arctic Gem?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Phthalo Turquoise Light is much more muted, cooler than Arctic Gem.

Can I use Arctic Gem instead of Phthalo Turquoise Light?

The same difference applies the other way round: Arctic Gem is much more saturated, 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.