Mars Red vs Red Orange

How close are they?

ΔE 17.4, loose: different enough that you should pick on purpose. Red Orange is a little lighter, warmer than Mars Red.

Can I use Red Orange instead of Mars Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Red Orange is a little lighter, warmer than Mars Red.

Can I use Mars Red instead of Red Orange?

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