ΔE 14.5, fair: the same colour family, but not a straight swap. Heartfire is a little lighter, cooler than Gryph-Hound Orange. They are different kinds of paint: Citadel Gryph-Hound Orange is a one-coat contrast-style paint and Formula P3 Heartfire 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 15). Heartfire is a little lighter, cooler than Gryph-Hound Orange. They are different kinds of paint.
The same difference applies the other way round: Gryph-Hound Orange is a little darker, 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 Gryph-Hound Orange on the colour wheel See Heartfire on the colour wheel
All matches: Gryph-Hound Orange equivalents · Heartfire equivalents