ΔE 20.0, loose: different enough that you should pick on purpose. Cobalt Alchemy is much more saturated than Broadsword Silver.
Can I use Cobalt Alchemy instead of Broadsword Silver?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Cobalt Alchemy is much more saturated than Broadsword Silver.
Can I use Broadsword Silver instead of Cobalt Alchemy?
The same difference applies the other way round: Broadsword Silver is much more muted.
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.