Moonstone Grey vs Sombre Grey

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Sombre Grey is a little darker, much more saturated than Moonstone Grey.

Can I use Sombre Grey instead of Moonstone Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Sombre Grey is a little darker, much more saturated than Moonstone Grey.

Can I use Moonstone Grey instead of Sombre Grey?

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