Fog Grey vs Ashen Blue

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Ashen Blue is noticeably darker, cooler than Fog Grey.

Can I use Ashen Blue instead of Fog Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ashen Blue is noticeably darker, cooler than Fog Grey.

Can I use Fog Grey instead of Ashen Blue?

The same difference applies the other way round: Fog Grey is noticeably lighter, warmer.

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.