ΔE 15.4, loose: different enough that you should pick on purpose. Phalanx Yellow is much more saturated than Purulent Yellow.
Can I use Phalanx Yellow instead of Purulent Yellow?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Phalanx Yellow is much more saturated than Purulent Yellow.
Can I use Purulent Yellow instead of Phalanx Yellow?
The same difference applies the other way round: Purulent Yellow is much more muted.
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.