Bloodmoon Red vs Martian Orange

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Martian Orange is a little darker, much more muted than Bloodmoon Red.

Can I use Martian Orange instead of Bloodmoon Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Martian Orange is a little darker, much more muted than Bloodmoon Red.

Can I use Bloodmoon Red instead of Martian Orange?

The same difference applies the other way round: Bloodmoon Red is a little lighter, 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.