Flag Red vs Inferno Orange

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Inferno Orange is noticeably lighter, more muted than Flag Red.

Can I use Inferno Orange instead of Flag Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Inferno Orange is noticeably lighter, more muted than Flag Red.

Can I use Flag Red instead of Inferno Orange?

The same difference applies the other way round: Flag Red is noticeably darker, 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.