Fog Grey vs Moonstone Grey

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Moonstone Grey is a little lighter, much more muted than Fog Grey.

Can I use Moonstone Grey instead of Fog Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Moonstone Grey is a little lighter, much more muted than Fog Grey.

Can I use Fog Grey instead of Moonstone Grey?

The same difference applies the other way round: Fog Grey is a little darker, much 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.