ΔE 5.2, very close: most people won’t see a difference side by side. Dark Grey RLM42 is a little lighter than Mechanicus Standard Grey.
Can I use Dark Grey RLM42 instead of Mechanicus Standard Grey?
Yes, for most jobs. Very close match (ΔE 5). Dark Grey RLM42 is a little lighter than Mechanicus Standard Grey.
Can I use Mechanicus Standard Grey instead of Dark Grey RLM42?
The same difference applies the other way round: Mechanicus Standard Grey is a little darker.
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.