ΔE 12.7, fair: the same colour family, but not a straight swap. The Fang is more saturated, warmer than Russian Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). The Fang is more saturated, warmer than Russian Blue.
The same difference applies the other way round: Russian Blue is more muted, 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 Russian Blue on the colour wheel See The Fang on the colour wheel
All matches: Russian Blue equivalents · The Fang equivalents