Ahriman Blue vs Blue Mountain

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Blue Mountain is a little darker, more muted, warmer than Ahriman Blue.

Can I use Blue Mountain instead of Ahriman Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Blue Mountain is a little darker, more muted, warmer than Ahriman Blue.

Can I use Ahriman Blue instead of Blue Mountain?

The same difference applies the other way round: Ahriman Blue is a little lighter, more saturated, cooler.

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.