ΔE 21.0, loose: different enough that you should pick on purpose. Purple is a little darker, much more saturated, warmer than Blue Mountain.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Purple is a little darker, much more saturated, warmer than Blue Mountain.
The same difference applies the other way round: Blue Mountain is a little lighter, much more muted, 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.
See Blue Mountain on the colour wheel See Purple on the colour wheel
All matches: Blue Mountain equivalents · Purple equivalents