Iron Wolf vs Terror Blue-Grey

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Terror Blue-Grey is a little darker, warmer than Iron Wolf.

Can I use Terror Blue-Grey instead of Iron Wolf?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Terror Blue-Grey is a little darker, warmer than Iron Wolf.

Can I use Iron Wolf instead of Terror Blue-Grey?

The same difference applies the other way round: Iron Wolf is a little lighter, cooler.

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.