Mechanicus Standard Grey vs Terror Blue-Grey

How close are they?

ΔE 6.6, close: similar, with a visible difference side by side. Terror Blue-Grey is a little lighter, more saturated than Mechanicus Standard Grey.

Can I use Terror Blue-Grey instead of Mechanicus Standard Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Terror Blue-Grey is a little lighter, more saturated than Mechanicus Standard Grey.

Can I use Mechanicus Standard Grey instead of Terror Blue-Grey?

The same difference applies the other way round: Mechanicus Standard Grey is a little darker, more muted.

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.