Per: http://msdn.microsoft.com/en-us/library/aa369212(v=vs.85).aspx
"""The Identifier data type is a text string. Identifiers may contain the
ASCII characters A-Z (a-z), digits, underscores (_), or periods (.). However, every identifier must begin with either a letter or an underscore."""

So the spec would say that colons are NOT allowed. Editing some entries in the File table of an MSI (using Orca from the MSI SDK) and running the validation confirms that.

All the following were flagged as errors during validation:
'KDiff3EXE;"ASDF@#$', 'chmFile-', 'pdfFile(', 'hgbook]', 'TortoisePlinkEXE]', 'Hg.Cämd', 'merg:epatterns.rc'

I think that this issue should be fixed when issue2694 is fixed. (I would like to put together some tests and patch for that :)