Ironbreaker vs Sharkfin Blue

How close are they?

ΔE 29.5, loose: different enough that you should pick on purpose. Sharkfin Blue is noticeably lighter, more saturated than Ironbreaker.

Can I use Sharkfin Blue instead of Ironbreaker?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Sharkfin Blue is noticeably lighter, more saturated than Ironbreaker.

Can I use Ironbreaker instead of Sharkfin Blue?

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