ΔE 13.9, fair: the same colour family, but not a straight swap. Nightshade Purple is noticeably darker, more saturated than Mechanicus Standard Grey.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Nightshade Purple is noticeably darker, more saturated than Mechanicus Standard Grey.
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.
See Mechanicus Standard Grey on the colour wheel See Nightshade Purple on the colour wheel
All matches: Mechanicus Standard Grey equivalents · Nightshade Purple equivalents