True Brass vs Martian Green

How close are they?

ΔE 26.4, loose: different enough that you should pick on purpose. Martian Green is a little darker, cooler than True Brass.

Can I use Martian Green instead of True Brass?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Martian Green is a little darker, cooler than True Brass.

Can I use True Brass instead of Martian Green?

The same difference applies the other way round: True Brass is a little lighter, warmer.

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.