Ruby vs Scarlet

How close are they?

ΔE 11.4, fair: the same colour family, but not a straight swap. Scarlet is a little darker, more saturated than Ruby.

Can I use Scarlet instead of Ruby?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Scarlet is a little darker, more saturated than Ruby.

Can I use Ruby instead of Scarlet?

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

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.