Mechanicus Standard Grey vs Huldra Blue

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Huldra Blue is a little darker than Mechanicus Standard Grey.

Can I use Huldra Blue instead of Mechanicus Standard Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Huldra Blue is a little darker than Mechanicus Standard Grey.

Can I use Mechanicus Standard Grey instead of Huldra Blue?

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.