Astorath Red vs Flat Red

How close are they?

ΔE 23.1, loose: different enough that you should pick on purpose. Flat Red is a little lighter, much more saturated than Astorath Red.

Can I use Flat Red instead of Astorath Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Flat Red is a little lighter, much more saturated than Astorath Red.

Can I use Astorath Red instead of Flat Red?

The same difference applies the other way round: Astorath Red is a little darker, much 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.