Mortarion Green Clear vs Ethereal Green

How close are they?

ΔE 19.7, loose: different enough that you should pick on purpose. Ethereal Green is a little darker, much more muted than Mortarion Green Clear.

Can I use Ethereal Green instead of Mortarion Green Clear?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ethereal Green is a little darker, much more muted than Mortarion Green Clear.

Can I use Mortarion Green Clear instead of Ethereal Green?

The same difference applies the other way round: Mortarion Green Clear is a little lighter, much more saturated.

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.