ΔE 5.7, very close: most people won’t see a difference side by side. Blue FS15044 is a little darker than Mechanicus Standard Grey.
Can I use Blue FS15044 instead of Mechanicus Standard Grey?
Yes, for most jobs. Very close match (ΔE 6). Blue FS15044 is a little darker than Mechanicus Standard Grey.
Can I use Mechanicus Standard Grey instead of Blue FS15044?
The same difference applies the other way round: Mechanicus Standard Grey is a little lighter.
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.