There is no option setting which one could modify, leaving the question as to whether there is a registry setting one could use.
Unfortunately, there is no single entry in registry that determines the default color of all markups. If Autodesk Design Review 2009 is installed on a clean machine, the red color (default) is determined by Autodesk Design Review 2009. However, once user starts to use the application, all format settings (color, line weight and so on, including all default settings) are captured in a XML file (user data) in the application data area.
Here is an example where users would find the file: C:\Documents and Settings\user\Application Data\Autodesk\Autodesk Design Review\10.0\userdata\woof_toolformats.xml
Said all that, users who do know how to edit an XML file successfully, could modify the file content and create the default color setting they wish to see.