Greatcoat Green vs Dark Yellow

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Dark Yellow is a little lighter, more saturated than Greatcoat Green.

Can I use Dark Yellow instead of Greatcoat Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Dark Yellow is a little lighter, more saturated than Greatcoat Green.

Can I use Greatcoat Green instead of Dark Yellow?

The same difference applies the other way round: Greatcoat Green is a little darker, 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.