Paints labelled as mars colours are made from synthetic iron oxide; the English Red, Indian Red and Yellow Ochre listed may actually be made from versions of these pigments already (PR101, PY42*).
Although it's not as opaque usually Burnt Umber is arguably a little more useful than Mars Brown, but these colours can vary a lot from brand to brand.
Einion
*I should mention, not all synthetic iron oxides are opaque - for both of the pigment codes listed the paint could be all the way from very opaque to extremely transparent, or somewhere in between.