Fire Opal vs Ruby Red

How close are they?

ΔE 27.1, loose: different enough that you should pick on purpose. Ruby Red is a little darker, much more saturated, cooler than Fire Opal.

Can I use Ruby Red instead of Fire Opal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Ruby Red is a little darker, much more saturated, cooler than Fire Opal.

Can I use Fire Opal instead of Ruby Red?

The same difference applies the other way round: Fire Opal is a little lighter, much more muted, 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.