Cygnus Yellow vs Lemon Yellow

How close are they?

ΔE 9.7, close: similar, with a visible difference side by side. Lemon Yellow is a little darker, more muted than Cygnus Yellow.

Can I use Lemon Yellow instead of Cygnus Yellow?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Lemon Yellow is a little darker, more muted than Cygnus Yellow.

Can I use Cygnus Yellow instead of Lemon Yellow?

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

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.