Ember Orange vs Flat Yellow

How close are they?

ΔE 24.5, loose: different enough that you should pick on purpose. Flat Yellow is noticeably lighter, cooler than Ember Orange.

Can I use Flat Yellow instead of Ember Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Flat Yellow is noticeably lighter, cooler than Ember Orange.

Can I use Ember Orange instead of Flat Yellow?

The same difference applies the other way round: Ember Orange is noticeably darker, 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.