Spiritstone Red vs Primer Red Oxide

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Primer Red Oxide is a little darker, much more muted than Spiritstone Red.

Can I use Primer Red Oxide instead of Spiritstone Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Primer Red Oxide is a little darker, much more muted than Spiritstone Red.

Can I use Spiritstone Red instead of Primer Red Oxide?

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