ΔE 8.1, close: similar, with a visible difference side by side. Flat Yellow is more muted than Warm Yellow. They are different kinds of paint: Pro Acryl Warm Yellow is a one-coat contrast-style paint and Vallejo Flat Yellow is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
✓ Herrick Games paint comparison chart lists these two as equivalents.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Yellow is more muted than Warm Yellow. They are different kinds of paint.
The same difference applies the other way round: Warm Yellow is more saturated.
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 Warm Yellow on the colour wheel See Flat Yellow on the colour wheel
All matches: Warm Yellow equivalents · Flat Yellow equivalents