ΔE 25.3, loose: different enough that you should pick on purpose. Metallic Brown is much more saturated than Castellax Bronze.
Can I use Metallic Brown instead of Castellax Bronze?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Metallic Brown is much more saturated than Castellax Bronze.
Can I use Castellax Bronze instead of Metallic Brown?
The same difference applies the other way round: Castellax Bronze 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.