Ultramarines Blue vs Dark Purple

How close are they?

ΔE 21.4, loose: different enough that you should pick on purpose. Dark Purple is a little darker, more muted, warmer than Ultramarines Blue.

Can I use Dark Purple instead of Ultramarines Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Purple is a little darker, more muted, warmer than Ultramarines Blue.

Can I use Ultramarines Blue instead of Dark Purple?

The same difference applies the other way round: Ultramarines Blue is a little lighter, 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.