Mechanicus Standard Grey vs Dark Purple

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Dark Purple is noticeably darker, more saturated than Mechanicus Standard Grey.

Can I use Dark Purple instead of Mechanicus Standard Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Dark Purple is noticeably darker, more saturated than Mechanicus Standard Grey.

Can I use Mechanicus Standard Grey instead of Dark Purple?

The same difference applies the other way round: Mechanicus Standard Grey is noticeably lighter, more muted.

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.