Dark Jade vs Light Emerald

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Light Emerald is noticeably lighter, more saturated than Dark Jade.

Can I use Light Emerald instead of Dark Jade?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Light Emerald is noticeably lighter, more saturated than Dark Jade.

Can I use Dark Jade instead of Light Emerald?

The same difference applies the other way round: Dark Jade is noticeably darker, 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.