Scarlet Red vs Phoenix Orange

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Phoenix Orange is a little darker, much more muted, warmer than Scarlet Red.

Can I use Phoenix Orange instead of Scarlet Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Phoenix Orange is a little darker, much more muted, warmer than Scarlet Red.

Can I use Scarlet Red instead of Phoenix Orange?

The same difference applies the other way round: Scarlet Red is a little lighter, much more saturated, 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.