Daemonic Yellow vs Cygnus Yellow

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Cygnus Yellow is a little lighter, cooler than Daemonic Yellow.

Can I use Cygnus Yellow instead of Daemonic Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Cygnus Yellow is a little lighter, cooler than Daemonic Yellow.

Can I use Daemonic Yellow instead of Cygnus Yellow?

The same difference applies the other way round: Daemonic Yellow is a little 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.