Cygnus Yellow vs Yellow Green

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Yellow Green is a little darker, cooler than Cygnus Yellow.

Can I use Yellow Green instead of Cygnus Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Yellow Green is a little darker, cooler than Cygnus Yellow.

Can I use Cygnus Yellow instead of Yellow Green?

The same difference applies the other way round: Cygnus Yellow is a little lighter, 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.