Mustard Yellow vs Flat Yellow

How close are they?

ΔE 13.4, fair: the same colour family, but not a straight swap. Flat Yellow is a little lighter, more saturated, warmer than Mustard Yellow.

Can I use Flat Yellow instead of Mustard Yellow?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Flat Yellow is a little lighter, more saturated, warmer than Mustard Yellow.

Can I use Mustard Yellow instead of Flat Yellow?

The same difference applies the other way round: Mustard Yellow is a little darker, more muted, 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.