Grey-Blue vs Violet Coven

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Violet Coven is more saturated, warmer than Grey-Blue.

Can I use Violet Coven instead of Grey-Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Violet Coven is more saturated, warmer than Grey-Blue.

Can I use Grey-Blue instead of Violet Coven?

The same difference applies the other way round: Grey-Blue is more muted, 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.