Spiritstone Red vs Red Iron Oxide

How close are they?

ΔE 20.1, loose: different enough that you should pick on purpose. Red Iron Oxide is a little darker, much more muted, warmer than Spiritstone Red.

Can I use Red Iron Oxide instead of Spiritstone Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Red Iron Oxide is a little darker, much more muted, warmer than Spiritstone Red.

Can I use Spiritstone Red instead of Red Iron Oxide?

The same difference applies the other way round: Spiritstone 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.