ΔE 21.4, loose: different enough that you should pick on purpose. Flat Yellow is a little lighter, more saturated, cooler than Moonbeam Yellow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Flat Yellow is a little lighter, more saturated, cooler than Moonbeam Yellow.
The same difference applies the other way round: Moonbeam Yellow is a little darker, 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.
See Moonbeam Yellow on the colour wheel See Flat Yellow on the colour wheel
All matches: Moonbeam Yellow equivalents · Flat Yellow equivalents