Having read this specific topic, and the related topic in the Scripts & Popups section (I believe it was), it appears to me that the OP is actually looking for a command to be able to change any part of the section in the Address Book that relates to highlighting, not just color.
Examples (and to the OP, please correct me if I'm mistaken)
/highlight add <nick/address>
/highlight match <nick/message/both>
/highlight Case <on/off>
/highlight Regex <on/off>
/highlight Color <mIRC color code>
/highlight Sound <sound file>
/highlight Flash <number of times, 0 for off>
/highlight Tip <time, 0 for off>
/highlight messsage <message>
and, in my opinion, a method of being able to combine the various switches and their respective settings, something like how /filter is configured.
