Ferocious Green vs Putrid Green

How close are they?

ΔE 29.1, loose: different enough that you should pick on purpose. Putrid Green is noticeably darker, more muted, warmer than Ferocious Green.

Can I use Putrid Green instead of Ferocious Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Putrid Green is noticeably darker, more muted, warmer than Ferocious Green.

Can I use Ferocious Green instead of Putrid Green?

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