Dark Leather vs Mechanicus Standard Grey

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Mechanicus Standard Grey is a little lighter than Dark Leather.

Can I use Mechanicus Standard Grey instead of Dark Leather?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Mechanicus Standard Grey is a little lighter than Dark Leather.

Can I use Dark Leather instead of Mechanicus Standard Grey?

The same difference applies the other way round: Dark Leather 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.