Arctic Gem vs Faded Ultramarine

How close are they?

ΔE 18.8, loose: different enough that you should pick on purpose. Faded Ultramarine is a little lighter, more muted, warmer than Arctic Gem.

Can I use Faded Ultramarine instead of Arctic Gem?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Faded Ultramarine is a little lighter, more muted, warmer than Arctic Gem.

Can I use Arctic Gem instead of Faded Ultramarine?

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