Pyrrhic Dawn vs Flat Yellow

How close are they?

ΔE 27.6, loose: different enough that you should pick on purpose. Flat Yellow is a little lighter, much more saturated, cooler than Pyrrhic Dawn.

Can I use Flat Yellow instead of Pyrrhic Dawn?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Flat Yellow is a little lighter, much more saturated, cooler than Pyrrhic Dawn.

Can I use Pyrrhic Dawn instead of Flat Yellow?

The same difference applies the other way round: Pyrrhic Dawn is a little darker, 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.