Shadow Thorn vs Mirage Blue

How close are they?

ΔE 6.6, close: similar, with a visible difference side by side. Mirage Blue is more muted, warmer than Shadow Thorn.

Can I use Mirage Blue instead of Shadow Thorn?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Mirage Blue is more muted, warmer than Shadow Thorn.

Can I use Shadow Thorn instead of Mirage Blue?

The same difference applies the other way round: Shadow Thorn is more saturated, 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.