ΔE 11.6, fair: the same colour family, but not a straight swap. Mustard Yellow is more muted, cooler than Nazdreg Yellow. They are different kinds of paint: Citadel Nazdreg Yellow is a one-coat contrast-style paint and Tamiya Mustard Yellow is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Mustard Yellow is more muted, cooler than Nazdreg Yellow. They are different kinds of paint.
The same difference applies the other way round: Nazdreg Yellow is 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 Nazdreg Yellow on the colour wheel See Mustard Yellow on the colour wheel
All matches: Nazdreg Yellow equivalents · Mustard Yellow equivalents