Maybe the Syntax-Highlightning of a Comment-line needs a fix so that it ends at a colon.
The syntax highlighting already does that, unless you use double semi-colon. Double semi-colon disables the
colon problem.
Try typing a line with a colon and then toggle the comment, eg.
ld a,(hl) ; Comment:ld b,a
-->
ld a,(hl) ;; Comment:ld b,a