Rainforest vs Cygnus Yellow

How close are they?

ΔE 33.2, loose: different enough that you should pick on purpose. Cygnus Yellow is noticeably lighter, more saturated, warmer than Rainforest.

Can I use Cygnus Yellow instead of Rainforest?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 33). Cygnus Yellow is noticeably lighter, more saturated, warmer than Rainforest.

Can I use Rainforest instead of Cygnus Yellow?

The same difference applies the other way round: Rainforest is noticeably 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.