Deep Grey vs Mechanicus Standard Grey

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Mechanicus Standard Grey is noticeably lighter, more muted than Deep Grey.

Can I use Mechanicus Standard Grey instead of Deep Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Mechanicus Standard Grey is noticeably lighter, more muted than Deep Grey.

Can I use Deep Grey instead of Mechanicus Standard Grey?

The same difference applies the other way round: Deep Grey is noticeably 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.