Warm Orange vs Grunge Brown

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Grunge Brown is a little darker, more muted than Warm Orange.

Can I use Grunge Brown instead of Warm Orange?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Grunge Brown is a little darker, more muted than Warm Orange.

Can I use Warm Orange instead of Grunge Brown?

The same difference applies the other way round: Warm Orange is a little lighter, more saturated.

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.