Etherium Blue vs Faded Ultramarine

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Faded Ultramarine is noticeably darker, more saturated, warmer than Etherium Blue.

Can I use Faded Ultramarine instead of Etherium Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Faded Ultramarine is noticeably darker, more saturated, warmer than Etherium Blue.

Can I use Etherium Blue instead of Faded Ultramarine?

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