Ultramarine Blue vs Alaitoc Blue

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Alaitoc Blue is noticeably lighter, more saturated, cooler than Ultramarine Blue.

Can I use Alaitoc Blue instead of Ultramarine Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Alaitoc Blue is noticeably lighter, more saturated, cooler than Ultramarine Blue.

Can I use Ultramarine Blue instead of Alaitoc Blue?

The same difference applies the other way round: Ultramarine Blue is noticeably darker, 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.