Ferocious Green vs Iosan Green

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Iosan Green is a little darker, much more saturated than Ferocious Green.

Can I use Iosan Green instead of Ferocious Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Iosan Green is a little darker, much more saturated than Ferocious Green.

Can I use Ferocious Green instead of Iosan Green?

The same difference applies the other way round: Ferocious Green is a little lighter, much more muted.

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.