ΔE 11.7, fair: the same colour family, but not a straight swap. Russ Grey is noticeably darker than Runic Cobalt.
Can I use Russ Grey instead of Runic Cobalt?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Russ Grey is noticeably darker than Runic Cobalt.
Can I use Runic Cobalt instead of Russ Grey?
The same difference applies the other way round: Runic Cobalt 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.