Cygnus Yellow vs Deep Yellow

How close are they?

ΔE 19.3, loose: different enough that you should pick on purpose. Deep Yellow is a little darker, more muted, warmer than Cygnus Yellow.

Can I use Deep Yellow instead of Cygnus Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Deep Yellow is a little darker, more muted, warmer than Cygnus Yellow.

Can I use Cygnus Yellow instead of Deep Yellow?

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

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.