Ironbreaker vs Cobalt Alchemy

How close are they?

ΔE 20.4, loose: different enough that you should pick on purpose. Cobalt Alchemy is noticeably darker, much more saturated than Ironbreaker.

Can I use Cobalt Alchemy instead of Ironbreaker?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Cobalt Alchemy is noticeably darker, much more saturated than Ironbreaker.

Can I use Ironbreaker instead of Cobalt Alchemy?

The same difference applies the other way round: Ironbreaker is noticeably lighter, 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.