Not entirely clear on the question ... are you'all currently colorizing your events in your Event Monitor based on priority (the other option is event status)?  You might be able to check by looking at cad > StatusDisplay > HowEventColor [also, whoever built your DefaultAddin.resources.dll would need to verify the corresponding setting in the underlying DefaultEventMonitor.xaml, as both must be in synch to colorize by status].  If you are colorizing by priority though, are you cad > StatusDisplay > P1Color and P3Color values the same?  If so, you should be able to add an overide to change whichever one you want (if different, and if colorizing by priority, the colors should already be different).  Or, are you perhaps colorizing by event status, and you'd like to somehow flag an event color to be different when changing from a P3 to a P1?  If so, I'm not sure that could readily be accomplished without engaging some additional Services help to perhaps write an Add-In module that would interact with the Monitors and update as desired -- which, so far I've not personally seen much of, nor tried myself.  Guessing it'd take some considerable effort.  Hoping it's just that you're P1 and P3 colors are the same!!