ΔE 15.5, loose: different enough that you should pick on purpose. Lemon Yellow is a little darker, more muted, cooler than Flat Yellow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Lemon Yellow is a little darker, more muted, cooler than Flat Yellow.
The same difference applies the other way round: Flat Yellow is a little lighter, more saturated, 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.
See Flat Yellow on the colour wheel See Lemon Yellow on the colour wheel
All matches: Flat Yellow equivalents · Lemon Yellow equivalents