Phalanx Yellow vs Cygnus Yellow

How close are they?

ΔE 17.4, loose: different enough that you should pick on purpose. Cygnus Yellow is a little lighter, more saturated, cooler than Phalanx Yellow.

Can I use Cygnus Yellow instead of Phalanx Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Cygnus Yellow is a little lighter, more saturated, cooler than Phalanx Yellow.

Can I use Phalanx Yellow instead of Cygnus Yellow?

The same difference applies the other way round: Phalanx Yellow is a little darker, more muted, 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.