Ultramarine Blue vs Crystal Blue

How close are they?

ΔE 26.3, loose: different enough that you should pick on purpose. Crystal Blue is noticeably lighter, much more saturated than Ultramarine Blue.

Can I use Crystal Blue instead of Ultramarine Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Crystal Blue is noticeably lighter, much more saturated than Ultramarine Blue.

Can I use Ultramarine Blue instead of Crystal Blue?

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

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.