Protective Green 1920s-1930s vs Flat Green

How close are they?

ΔE 9.4, close: similar, with a visible difference side by side. Flat Green is a little lighter than Protective Green 1920s-1930s.

Can I use Flat Green instead of Protective Green 1920s-1930s?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Flat Green is a little lighter than Protective Green 1920s-1930s.

Can I use Protective Green 1920s-1930s instead of Flat Green?

The same difference applies the other way round: Protective Green 1920s-1930s is a little darker.

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.