ΔE 6.3, close: similar, with a visible difference side by side. Phalanx Yellow is more muted than Daemonic Yellow.
Can I use Phalanx Yellow instead of Daemonic Yellow?
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Phalanx Yellow is more muted than Daemonic Yellow.
Can I use Daemonic Yellow instead of Phalanx Yellow?
The same difference applies the other way round: Daemonic Yellow is 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.