ΔE 8.6, close: similar, with a visible difference side by side. Flat Red is a little darker, more saturated than Ruby Red.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Flat Red is a little darker, more saturated than Ruby Red.
The same difference applies the other way round: Ruby Red 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.
See Ruby Red on the colour wheel See Flat Red on the colour wheel
All matches: Ruby Red equivalents · Flat Red equivalents