Macragge Blue vs Dark Ultramarine

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Dark Ultramarine is much more saturated, warmer than Macragge Blue.

Can I use Dark Ultramarine instead of Macragge Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Dark Ultramarine is much more saturated, warmer than Macragge Blue.

Can I use Macragge Blue instead of Dark Ultramarine?

The same difference applies the other way round: Macragge Blue is much more muted, 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.