Resplendent Red vs Firethorn Brown

How close are they?

ΔE 17.0, loose: different enough that you should pick on purpose. Firethorn Brown is a little lighter, more saturated, warmer than Resplendent Red.

Can I use Firethorn Brown instead of Resplendent Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Firethorn Brown is a little lighter, more saturated, warmer than Resplendent Red.

Can I use Resplendent Red instead of Firethorn Brown?

The same difference applies the other way round: Resplendent Red is a little darker, more muted, cooler.

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.