Let Them Eat Cake vs Ultramarine Blue

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Ultramarine Blue is more muted, cooler than Let Them Eat Cake.

Can I use Ultramarine Blue instead of Let Them Eat Cake?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ultramarine Blue is more muted, cooler than Let Them Eat Cake.

Can I use Let Them Eat Cake instead of Ultramarine Blue?

The same difference applies the other way round: Let Them Eat Cake is 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.