Martian Green vs Greenish Gold

How close are they?

ΔE 27.9, loose: different enough that you should pick on purpose. Greenish Gold is noticeably lighter, more saturated, warmer than Martian Green.

Can I use Greenish Gold instead of Martian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Greenish Gold is noticeably lighter, more saturated, warmer than Martian Green.

Can I use Martian Green instead of Greenish Gold?

The same difference applies the other way round: Martian Green is noticeably darker, more muted, cooler.

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.