Mystic Green vs Ferocious Green

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Ferocious Green is noticeably darker, more saturated, cooler than Mystic Green.

Can I use Ferocious Green instead of Mystic Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ferocious Green is noticeably darker, more saturated, cooler than Mystic Green.

Can I use Mystic Green instead of Ferocious Green?

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