change highlight color

If you have assorted comments, use the filter to display only highlighters, then select them all to change them together. Right-click for Properties.

To set the default, right-click only one, not multiples—either in the document or in the Comments panel.

Yes, it can be done either manually (select all the comments in the list, right-click one of them, select Properties and change the fill color), or by using a script.