ΔE 26.8, loose: different enough that you should pick on purpose. Steampunk Copper is noticeably lighter than Castellax Bronze.
Can I use Steampunk Copper instead of Castellax Bronze?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Steampunk Copper is noticeably lighter than Castellax Bronze.
Can I use Castellax Bronze instead of Steampunk Copper?
The same difference applies the other way round: Castellax Bronze is noticeably darker.
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.