
ColorSchemes is only there for backward compatibility. In Office 2007 and later, you would use ThemeColorScheme.Load. That command requires the complete path to the color theme, which will include your username. So you would need to use VBA to get that information:
Sub SetColorTheme()
strFilePath = Environ("APPDATA") & "\Microsoft\Templates\Document Themes\Theme Colors\"
ActivePresentation.Designs(1).slideMaster.CustomLayouts(1).ThemeColorScheme.Load (strFilePath & "Custom 2.xml")
End Sub