Faded Ultramarine vs Magic Blue

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Magic Blue is a little darker, much more saturated, cooler than Faded Ultramarine.

Can I use Magic Blue instead of Faded Ultramarine?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Magic Blue is a little darker, much more saturated, cooler than Faded Ultramarine.

Can I use Faded Ultramarine instead of Magic Blue?

The same difference applies the other way round: Faded Ultramarine is a little lighter, much 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.