ΔE 16.6, loose: different enough that you should pick on purpose. Faded Ultramarine is much more saturated, warmer than Runic Cobalt.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Faded Ultramarine is much more saturated, warmer than Runic Cobalt.
The same difference applies the other way round: Runic Cobalt is much 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 Runic Cobalt on the colour wheel See Faded Ultramarine on the colour wheel
All matches: Runic Cobalt equivalents · Faded Ultramarine equivalents