Mars Yellow vs Grunge Brown

How close are they?

ΔE 7.9, close: similar, with a visible difference side by side. Grunge Brown is a little lighter, more saturated than Mars Yellow.

Can I use Grunge Brown instead of Mars Yellow?

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

Can I use Mars Yellow instead of Grunge Brown?

The same difference applies the other way round: Mars Yellow 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.