Frost Blue vs Moonstone Grey

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Moonstone Grey is noticeably darker, much more muted than Frost Blue.

Can I use Moonstone Grey instead of Frost Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Moonstone Grey is noticeably darker, much more muted than Frost Blue.

Can I use Frost Blue instead of Moonstone Grey?

The same difference applies the other way round: Frost Blue is noticeably lighter, 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.