Frenzy Orange vs Martian Orange

How close are they?

ΔE 23.1, loose: different enough that you should pick on purpose. Martian Orange is noticeably darker, much more muted, cooler than Frenzy Orange.

Can I use Martian Orange instead of Frenzy Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Martian Orange is noticeably darker, much more muted, cooler than Frenzy Orange.

Can I use Frenzy Orange instead of Martian Orange?

The same difference applies the other way round: Frenzy Orange is noticeably lighter, much more saturated, 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.