Health Red vs Martian Orange

How close are they?

ΔE 9.8, close: similar, with a visible difference side by side. Martian Orange is a little lighter, more muted than Health Red.

Can I use Martian Orange instead of Health Red?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Martian Orange is a little lighter, more muted than Health Red.

Can I use Health Red instead of Martian Orange?

The same difference applies the other way round: Health Red is a little darker, 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.