ΔE 13.7, fair: the same colour family, but not a straight swap. Fire Red is a little lighter, more saturated than Royal Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Fire Red is a little lighter, more saturated than Royal Brown.
The same difference applies the other way round: Royal Brown 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.
See Royal Brown on the colour wheel See Fire Red on the colour wheel
All matches: Royal Brown equivalents · Fire Red equivalents