Extra Dark Green vs Dark Reaper

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Dark Reaper is noticeably lighter, cooler than Extra Dark Green.

Can I use Dark Reaper instead of Extra Dark Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Dark Reaper is noticeably lighter, cooler than Extra Dark Green.

Can I use Extra Dark Green instead of Dark Reaper?

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