Re: can change message's color?

[Quoted] Try this:

Create an alert for your message box and define your color requirements in the property palette. Use the set_alert_property built in to change the message before displaying the alert, and the show_alert function to display the alert.

Here is sample code from the help doc:
DECLARE
   alert_id ALERT := Find_Alert('generic_alert');    dummy_var NUMBER;
BEGIN
   Set_Alert_Property(alert_id, ALERT_MESSAGE_TEXT, 'The product      you selected is not in stock');

• now show the alert dummy_var := Show_Alert(alert_id); END;