This is what I used to get top center red colored message in my spawn protection plugin :
This is the code for the message itsel and the 10.0 is the seconds it lasts , change that if you want.
Code:
set_hudmessage( 255, 0, 0, -1.0, -1.0, 0, 6.0, 10.0, 0.5, 1.5, 4 )
This is the message in between the quotes , change that :
Code:
show_hudmessage(0, "MESSAGE HERE")
Good luck.