Leafy Green vs Permanent Yellow

How close are they?

ΔE 49.0, loose: different enough that you should pick on purpose. Permanent Yellow is noticeably lighter, more saturated, warmer than Leafy Green.

Can I use Permanent Yellow instead of Leafy Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 49). Permanent Yellow is noticeably lighter, more saturated, warmer than Leafy Green.

Can I use Leafy Green instead of Permanent Yellow?

The same difference applies the other way round: Leafy Green 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.