The painted red strip around the outside of Roman Buildings

[Viventius]

They used red ochre for pigmentation. Also cinnabar and lead, which would be rather more expensive (and toxic!). Most of the red wall paint I've seen come up in excavations was red ochre. The red isn't as intense as cinnabar, but is sufficient.

There's information about the composition of wall paints here:

http://www.metmuseum.org/toah/hd/ropt/hd_ropt.htm