Field Green vs Sick Green

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. Sick Green is a little lighter, much more saturated than Field Green.

Can I use Sick Green instead of Field Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Sick Green is a little lighter, much more saturated than Field Green.

Can I use Field Green instead of Sick Green?

The same difference applies the other way round: Field Green is a little darker, much 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.