Dark Ultramarine vs Royal Blue

How close are they?

ΔE 20.9, loose: different enough that you should pick on purpose. Royal Blue is a little lighter, much more muted, cooler than Dark Ultramarine.

Can I use Royal Blue instead of Dark Ultramarine?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Royal Blue is a little lighter, much more muted, cooler than Dark Ultramarine.

Can I use Dark Ultramarine instead of Royal Blue?

The same difference applies the other way round: Dark Ultramarine is a little darker, 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.