ΔE 12.1, fair: the same colour family, but not a straight swap. Field Gray is noticeably darker than Stormfang.
Can I use Field Gray instead of Stormfang?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Field Gray is noticeably darker than Stormfang.
Can I use Stormfang instead of Field Gray?
The same difference applies the other way round: Stormfang is noticeably lighter.
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.