A colour described as 'Mars' means it is made from a synthetic iron oxide, in comparison to a traditional earth paint like an ochre or umber (generally a natural iron oxide with additional minerals) it is cleaner and more consistent.
Now there is a lot of good things to be said for natural earths and quite frankly I am a massive fan of them, which is why there is the widest range available in the UK on this site.
However they can struggle to keep up with more modern pigments in mixes due to their low tinting power, these synthetic earths with their greater tinting power and cleanliness can fair better. A longer explanation of the Mars colours can be found on the Mars Orange page.
John Singer Sargent used Mars colours which is a pretty good endorsement.
In masstone this is chocolate brown, with a reddish undertone which leads to warm, purplish greys as a tint.
Pigment - Synthetic Iron Oxide, PBr6
Vehicle - Linseed Oil, Consistency - Soft Butter