Cygnus Yellow vs Light Yellow

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Light Yellow is a little darker, much more muted, warmer than Cygnus Yellow.

Can I use Light Yellow instead of Cygnus Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Light Yellow is a little darker, much more muted, warmer than Cygnus Yellow.

Can I use Cygnus Yellow instead of Light Yellow?

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