Fire Giant Orange vs Scarlet Red

How close are they?

ΔE 37.4, loose: different enough that you should pick on purpose. Scarlet Red is noticeably darker, cooler than Fire Giant Orange.

Can I use Scarlet Red instead of Fire Giant Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 37). Scarlet Red is noticeably darker, cooler than Fire Giant Orange.

Can I use Fire Giant Orange instead of Scarlet Red?

The same difference applies the other way round: Fire Giant Orange is noticeably lighter, 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.