AMT-12 Dark Grey vs Mechanicus Standard Grey

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Mechanicus Standard Grey is a little lighter than AMT-12 Dark Grey.

Can I use Mechanicus Standard Grey instead of AMT-12 Dark Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Mechanicus Standard Grey is a little lighter than AMT-12 Dark Grey.

Can I use AMT-12 Dark Grey instead of Mechanicus Standard Grey?

The same difference applies the other way round: AMT-12 Dark Grey 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.