Phalanx Yellow vs Marduk Yellow

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Marduk Yellow is more saturated than Phalanx Yellow.

Can I use Marduk Yellow instead of Phalanx Yellow?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Marduk Yellow is more saturated than Phalanx Yellow.

Can I use Phalanx Yellow instead of Marduk Yellow?

The same difference applies the other way round: Phalanx Yellow is 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.