Sulfuric Yellow vs Flat Yellow

How close are they?

ΔE 23.8, loose: different enough that you should pick on purpose. Flat Yellow is a little darker, more muted, warmer than Sulfuric Yellow.

Can I use Flat Yellow instead of Sulfuric Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Flat Yellow is a little darker, more muted, warmer than Sulfuric Yellow.

Can I use Sulfuric Yellow instead of Flat Yellow?

The same difference applies the other way round: Sulfuric Yellow is a little lighter, more saturated, cooler.

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.