Troglodyte Blue vs Blue Mountain

How close are they?

ΔE 30.1, loose: different enough that you should pick on purpose. Blue Mountain is noticeably darker, much more muted, warmer than Troglodyte Blue.

Can I use Blue Mountain instead of Troglodyte Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Blue Mountain is noticeably darker, much more muted, warmer than Troglodyte Blue.

Can I use Troglodyte Blue instead of Blue Mountain?

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