Crystal Blue vs Ultramarine

How close are they?

ΔE 19.9, loose: different enough that you should pick on purpose. Ultramarine is a little darker, more saturated, warmer than Crystal Blue.

Can I use Ultramarine instead of Crystal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ultramarine is a little darker, more saturated, warmer than Crystal Blue.

Can I use Crystal Blue instead of Ultramarine?

The same difference applies the other way round: Crystal Blue is a little 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.