It could be simpler to use a different color for their name, like with new users having a green color: When a user comments more than once in the same subtree,

It could be simpler to use a different color for their name, like with new users having a green color: When a user comments more than once in the same subtree, change the username color in all their comments. Leave single commenters as grey. This should be pretty easy to do with a greasemonkey script.