Ultramarine Blue vs Gothic Blue

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Gothic Blue is a little darker, much more saturated, warmer than Ultramarine Blue.

Can I use Gothic Blue instead of Ultramarine Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Gothic Blue is a little darker, much more saturated, warmer than Ultramarine Blue.

Can I use Ultramarine Blue instead of Gothic Blue?

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