Grey-Blue vs Fenrisian Grey

How close are they?

ΔE 7.3, close: similar, with a visible difference side by side. Fenrisian Grey is a little darker, more muted than Grey-Blue.

Can I use Fenrisian Grey instead of Grey-Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Fenrisian Grey is a little darker, more muted than Grey-Blue.

Can I use Grey-Blue instead of Fenrisian Grey?

The same difference applies the other way round: Grey-Blue is a little lighter, more saturated.

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.