ΔE 14.0, fair: the same colour family, but not a straight swap. Old Copper is noticeably darker, warmer than Castellax Bronze.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Old Copper is noticeably darker, warmer than Castellax Bronze.
The same difference applies the other way round: Castellax Bronze is noticeably lighter, 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.
See Castellax Bronze on the colour wheel See Old Copper on the colour wheel
All matches: Castellax Bronze equivalents · Old Copper equivalents