ΔE 9.3, close: similar, with a visible difference side by side. Mechanicus Standard Grey is a little darker than US Army Helo Drab FS 34031.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Mechanicus Standard Grey is a little darker than US Army Helo Drab FS 34031.
The same difference applies the other way round: US Army Helo Drab FS 34031 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.
See US Army Helo Drab FS 34031 on the colour wheel See Mechanicus Standard Grey on the colour wheel
All matches: US Army Helo Drab FS 34031 equivalents · Mechanicus Standard Grey equivalents