I've used the following for a script on monthly metrics:

my $percent_format = $workbook->addformat();
$percent_format->set_num_format(
   '[Red][<=.96]0.0%;[Blue][<=.99]0.0%;[Color 10]0.0%');
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And then you use that format wherever a format is called for. Basically, it says if the value is less than or equal to .96, color it red, if it's less than or equal to .99, color it blue, otherwise, color it with 'Color 10' (which is green, IIRC). Moreover, it formats it as a percent with one place after the decimal point. When I wrote this, I remember there being some limit to how many conditions you could have. I could be wrong, though. HTH.