ΔE 13.3, fair: the same colour family, but not a straight swap. Faded P.R.U. Blue is noticeably darker than Stormfang.
Can I use Faded P.R.U. Blue instead of Stormfang?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Faded P.R.U. Blue is noticeably darker than Stormfang.
Can I use Stormfang instead of Faded P.R.U. Blue?
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.