Poisonous Cloud vs Serpent Eye Yellow

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Serpent Eye Yellow is more saturated, warmer than Poisonous Cloud.

Can I use Serpent Eye Yellow instead of Poisonous Cloud?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Serpent Eye Yellow is more saturated, warmer than Poisonous Cloud.

Can I use Poisonous Cloud instead of Serpent Eye Yellow?

The same difference applies the other way round: Poisonous Cloud is 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.