Poisonous Cloud vs Cygnus Yellow

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Cygnus Yellow is a little lighter, more saturated, warmer than Poisonous Cloud.

Can I use Cygnus Yellow instead of Poisonous Cloud?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Cygnus Yellow is a little lighter, more saturated, warmer than Poisonous Cloud.

Can I use Poisonous Cloud instead of Cygnus Yellow?

The same difference applies the other way round: Poisonous Cloud is a little darker, 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.