Mechanicus Standard Grey vs Rlyeh Grey

How close are they?

ΔE 9.4, close: similar, with a visible difference side by side. Rlyeh Grey is a little lighter than Mechanicus Standard Grey.

Can I use Rlyeh Grey instead of Mechanicus Standard Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Rlyeh Grey is a little lighter than Mechanicus Standard Grey.

Can I use Mechanicus Standard Grey instead of Rlyeh Grey?

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.