ΔE 20.1, loose: different enough that you should pick on purpose. Cobalt Alchemy is much more saturated than Leadbelcher.
Can I use Cobalt Alchemy instead of Leadbelcher?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Cobalt Alchemy is much more saturated than Leadbelcher.
Can I use Leadbelcher instead of Cobalt Alchemy?
The same difference applies the other way round: Leadbelcher 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.