Crystal Blue vs Faded Ultramarine

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Faded Ultramarine is noticeably lighter, much more muted, warmer than Crystal Blue.

Can I use Faded Ultramarine instead of Crystal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Faded Ultramarine is noticeably lighter, much more muted, warmer than Crystal Blue.

Can I use Crystal Blue instead of Faded Ultramarine?

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