Fire Orange vs Grunge Brown

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Grunge Brown is a little darker, much more muted than Fire Orange.

Can I use Grunge Brown instead of Fire Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Grunge Brown is a little darker, much more muted than Fire Orange.

Can I use Fire Orange instead of Grunge Brown?

The same difference applies the other way round: Fire Orange is a little lighter, much 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.