Incubi Darkness vs Ultramarine Shadow

How close are they?

ΔE 9.5, close: similar, with a visible difference side by side. Ultramarine Shadow is a little darker, warmer than Incubi Darkness.

Can I use Ultramarine Shadow instead of Incubi Darkness?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Ultramarine Shadow is a little darker, warmer than Incubi Darkness.

Can I use Incubi Darkness instead of Ultramarine Shadow?

The same difference applies the other way round: Incubi Darkness is a little lighter, 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.