Thunder Storm vs Ultramarine Shadow

How close are they?

ΔE 8.8, close: similar, with a visible difference side by side. Ultramarine Shadow is a little lighter, more saturated, cooler than Thunder Storm.

Can I use Ultramarine Shadow instead of Thunder Storm?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Ultramarine Shadow is a little lighter, more saturated, cooler than Thunder Storm.

Can I use Thunder Storm instead of Ultramarine Shadow?

The same difference applies the other way round: Thunder Storm is a little darker, more muted, 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.