Shadow Thorn vs Huldra Blue

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Huldra Blue is a little darker, much more muted, warmer than Shadow Thorn.

Can I use Huldra Blue instead of Shadow Thorn?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Huldra Blue is a little darker, much more muted, warmer than Shadow Thorn.

Can I use Shadow Thorn instead of Huldra Blue?

The same difference applies the other way round: Shadow Thorn is a little 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.