ΔE 19.7, loose: different enough that you should pick on purpose. Serpent Eye Yellow is a little darker, much more saturated, warmer than Pastel Yellow. They are different kinds of paint: Army Painter Pastel Yellow is a one-coat contrast-style paint and Two Thin Coats Serpent Eye Yellow is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Serpent Eye Yellow is a little darker, much more saturated, warmer than Pastel Yellow. They are different kinds of paint.
The same difference applies the other way round: Pastel Yellow is a little lighter, much more muted, cooler.
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.
See Pastel Yellow on the colour wheel See Serpent Eye Yellow on the colour wheel
All matches: Pastel Yellow equivalents · Serpent Eye Yellow equivalents