ΔE 15.1, loose: different enough that you should pick on purpose. Raw Umber is a little darker than Mechanicus Standard Grey.
Can I use Raw Umber instead of Mechanicus Standard Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Raw Umber is a little darker than Mechanicus Standard Grey.
Can I use Mechanicus Standard Grey instead of Raw Umber?
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.