Dark Yellow vs Cayman Green

How close are they?

ΔE 19.0, loose: different enough that you should pick on purpose. Cayman Green is noticeably darker, more muted than Dark Yellow.

Can I use Cayman Green instead of Dark Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Cayman Green is noticeably darker, more muted than Dark Yellow.

Can I use Dark Yellow instead of Cayman Green?

The same difference applies the other way round: Dark Yellow is noticeably lighter, more saturated.

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.