That makes two of us... and as I do not have Flash installed I can not see the effect.
But going on your final statement:
"I tried with a .png that does work, but does anyone has an other way to achieve this?"
How about this approach: button_JHSv1.hype.zip (2.1 MB)
(3) green translucent rectangle elements are placed in the "3-Btn Hilite" group inside your "Button" symbol.
"On Mouse Over" & "On Mouse Leave" trigger the timeline called "Green Hilite" which animates the opacity from 0-100 ("Mouse Over") to 100-0 ("Mouse Leave").
You can of course add other code to the "3-Btn Hilite" group for click events, etc.
Addendum: In addition to the entire "3-Btn Hilite" group generating events each rectangle could have adjustable properties and respond to individual mouse events or other triggers.