Ultramarines Blue vs Mystic Blue

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Mystic Blue is much more muted, cooler than Ultramarines Blue.

Can I use Mystic Blue instead of Ultramarines Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Mystic Blue is much more muted, cooler than Ultramarines Blue.

Can I use Ultramarines Blue instead of Mystic Blue?

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