Dark Emerald vs Red Goblin

How close are they?

ΔE 30.2, loose: different enough that you should pick on purpose. Red Goblin is noticeably lighter, more saturated, cooler than Dark Emerald.

Can I use Red Goblin instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Red Goblin is noticeably lighter, more saturated, cooler than Dark Emerald.

Can I use Dark Emerald instead of Red Goblin?

The same difference applies the other way round: Dark Emerald is noticeably darker, more muted, warmer.

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.