There isn't any way to change the flash colour unless you modify the java script(coding) of the program.
what you can do is add the tint effect to your character and change the redness of it to 255 and then in your event add an event that disables the effect at the start of the layout, and when ever you want him to flash just add the action:
set effect enabled
wait 0.5 seconds
set effect disabled
wait 0.5 seconds
set effect enabled
wait 0.5 seconds
set effect disabled
wait 0.5 seconds
and that shall make your character flash