Reading the source: even though there is "a font format for cell comments" defined, with hard-coded attributes, it's probably never used. Comments' text font, size and color appear to be even more deeply hard-coded. So, ugly solution would be to hack source, or re-define a sub in Excel::Writer::XLSX::Package::Comments package.

As to color index "81", I'm not sure but it looks like 64 and above is "default" i.e. black. Why 81 then? Don't know. To use different color for different comments, other than post-process XLSX files (which are ZIP archives, as you of course know), I have no other solutions.