Elysian Green vs Orc Green

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Orc Green is a little darker, more saturated, cooler than Elysian Green.

Can I use Orc Green instead of Elysian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Orc Green is a little darker, more saturated, cooler than Elysian Green.

Can I use Elysian Green instead of Orc Green?

The same difference applies the other way round: Elysian Green is a little lighter, 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.