ΔE 11.3, fair: the same colour family, but not a straight swap. Flat Red is a little lighter, cooler than Fiery Vermilion.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Flat Red is a little lighter, cooler than Fiery Vermilion.
The same difference applies the other way round: Fiery Vermilion is a little darker, warmer.
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 Fiery Vermilion on the colour wheel See Flat Red on the colour wheel
All matches: Fiery Vermilion equivalents · Flat Red equivalents