ΔE 26.9, loose: different enough that you should pick on purpose. Magnesium is noticeably lighter, much more muted than True Brass.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Magnesium is noticeably lighter, much more muted than True Brass.
The same difference applies the other way round: True Brass is noticeably darker, much more saturated.
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.
See True Brass on the colour wheel See Magnesium on the colour wheel
All matches: True Brass equivalents · Magnesium equivalents