Modern Spanish Green vs Fern Green

How close are they?

ΔE 6.7, close: similar, with a visible difference side by side. Fern Green is cooler than Modern Spanish Green.

Can I use Fern Green instead of Modern Spanish Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Fern Green is cooler than Modern Spanish Green.

Can I use Modern Spanish Green instead of Fern Green?

The same difference applies the other way round: Modern Spanish Green is 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.