Black Forest vs Mechanicus Standard Grey

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Mechanicus Standard Grey is a little lighter, more muted than Black Forest.

Can I use Mechanicus Standard Grey instead of Black Forest?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Mechanicus Standard Grey is a little lighter, more muted than Black Forest.

Can I use Black Forest instead of Mechanicus Standard Grey?

The same difference applies the other way round: Black Forest is a little darker, 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.