Space Purple vs Mechanicus Standard Grey

How close are they?

ΔE 19.6, loose: different enough that you should pick on purpose. Mechanicus Standard Grey is a little lighter, much more muted than Space Purple.

Can I use Mechanicus Standard Grey instead of Space Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Mechanicus Standard Grey is a little lighter, much more muted than Space Purple.

Can I use Space Purple instead of Mechanicus Standard Grey?

The same difference applies the other way round: Space Purple is a little darker, much 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.