Light Ultramarine Blue vs Elfic Blue

How close are they?

ΔE 24.4, loose: different enough that you should pick on purpose. Elfic Blue is much more muted, cooler than Light Ultramarine Blue.

Can I use Elfic Blue instead of Light Ultramarine Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Elfic Blue is much more muted, cooler than Light Ultramarine Blue.

Can I use Light Ultramarine Blue instead of Elfic Blue?

The same difference applies the other way round: Light Ultramarine Blue 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.