Freezing Moon vs Ahriman Blue

How close are they?

ΔE 21.6, loose: different enough that you should pick on purpose. Ahriman Blue is noticeably darker, more saturated, cooler than Freezing Moon.

Can I use Ahriman Blue instead of Freezing Moon?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Ahriman Blue is noticeably darker, more saturated, cooler than Freezing Moon.

Can I use Freezing Moon instead of Ahriman Blue?

The same difference applies the other way round: Freezing Moon is noticeably lighter, more muted, 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.