It looks like the Colors array is resistant to change:

    #!/usr/bin/perl -wl


    use strict;
    use Cwd;
    use Win32::OLE;
    use Win32::OLE::Const 'Microsoft Excel';


    my $application = Win32::OLE->new("Excel.Application");
    my $workbook    = $application->Workbooks->Add;
    my $worksheet   = $workbook->Worksheets(1);


    print "Before: ", $workbook->Colors->[0];
    print "Trying: ", $workbook->Colors->[0] = 255;
    print "After:  ", $workbook->Colors->[0];



    $workbook->SaveAs({
                        FileName   => cwd() . '/win32ole.xls', 
                        FileFormat => xlNormal,
                      });
    $workbook->Close;

    __END__

    Prints:

    Before: 0
    Trying: 255
    After:  0
[download]

But I don't know why. Probably, Win32::OLE sets it as read-only but it is definitely writeable from VBA.

--
John.