Medieval Forest vs Bestial Green

How close are they?

ΔE 19.5, loose: different enough that you should pick on purpose. Bestial Green is a little darker, more muted, warmer than Medieval Forest.

Can I use Bestial Green instead of Medieval Forest?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Bestial Green is a little darker, more muted, warmer than Medieval Forest.

Can I use Medieval Forest instead of Bestial Green?

The same difference applies the other way round: Medieval Forest 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.