The dialog shows the nick colors rule colored to be the color it would trigger for those nicks, which is helpful, except for the fact that the background color is taken from control panel settings instead of using the color that nicks will actually be presented against.
If the background is black, people will be using colors like yellow or white or light-green - and if background is white people will use colors like blue. However, if mIRC is set to have a black background and the system colors have a white background, this causes rules to be presented as white-on-white, yellow-on-white, etc.
If possible, it would be great if this dialog would use the same background RGB against which the nicks would be presented. Since it's possible for nicklist and channel backgrounds to be different shades, I'm guessing that $color(listbox background) should be the one used.