ΔE 20.5, loose: different enough that you should pick on purpose. Top Brass is noticeably lighter than Skullcrusher Brass.
Can I use Top Brass instead of Skullcrusher Brass?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Top Brass is noticeably lighter than Skullcrusher Brass.
Can I use Skullcrusher Brass instead of Top Brass?
The same difference applies the other way round: Skullcrusher Brass 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.