Terror Blue-Grey vs Grey Blue

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Grey Blue is noticeably lighter, more saturated than Terror Blue-Grey.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Grey Blue is noticeably lighter, more saturated than Terror Blue-Grey.

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

The same difference applies the other way round: Terror Blue-Grey is noticeably 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.