Red Purple vs Flat Red

How close are they?

ΔE 23.9, loose: different enough that you should pick on purpose. Flat Red is a little darker, warmer than Red Purple.

Can I use Flat Red instead of Red Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Flat Red is a little darker, warmer than Red Purple.

Can I use Red Purple instead of Flat Red?

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