Ultramarine Shadow vs Steel Blue

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Steel Blue is a little lighter than Ultramarine Shadow.

Can I use Steel Blue instead of Ultramarine Shadow?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Steel Blue is a little lighter than Ultramarine Shadow.

Can I use Ultramarine Shadow instead of Steel Blue?

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

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.