Ant,
Did you see my over-the-top reply to Richard Budig's thread on Indian Red and Terra Rosa? I posted it yesterday. More info than you would ever want to know!
http://forum.portraitartist.com/showthread.php?t=5867
A warm color like Terra Rosa can be a good indicator of what is in the white. As you know, lead/flake white makes a much warmer tint than Titanium/Zinc does.
Garth