From my own experience I don't think there is a way, however, if you were to replace the calc. condition you used for the warning with a show condition to show an object then if you create a text box with your changes that you desire you can optimize it. So you have a quick workaround! Quick little thing though even when the object it is hidden it's still being calculated by the system if you create a macro to calculate only objects that are physically shown then it won't affect your processing time. However, in most cases no need to do this.
From my experience i don't thing so this would be possible to change the font color of any error message. Yes you can use any text object over the chart and make its layout properties as a top and programmed the code to view error message.
