Medieval Forest vs Bright Jade

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Bright Jade is noticeably lighter, much more saturated than Medieval Forest.

Can I use Bright Jade instead of Medieval Forest?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Bright Jade is noticeably lighter, much more saturated than Medieval Forest.

Can I use Medieval Forest instead of Bright Jade?

The same difference applies the other way round: Medieval Forest is noticeably darker, much more muted.

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.