Shadow Grey vs Mechanicus Standard Grey

How close are they?

ΔE 8.4, close: similar, with a visible difference side by side. Mechanicus Standard Grey is more muted than Shadow Grey.

Can I use Mechanicus Standard Grey instead of Shadow Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Mechanicus Standard Grey is more muted than Shadow Grey.

Can I use Shadow Grey instead of Mechanicus Standard Grey?

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

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.