Mustard Yellow vs Cayman Green

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Cayman Green is much more muted, cooler than Mustard Yellow.

Can I use Cayman Green instead of Mustard Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Cayman Green is much more muted, cooler than Mustard Yellow.

Can I use Mustard Yellow instead of Cayman Green?

The same difference applies the other way round: Mustard Yellow is much more saturated, warmer.

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.