Adding flashing light effect to an object

create a circle shape with red fill

from Effect Controls, you have to keyframe the Opacity value,

click the stopwatch next to Opacity, should be set to 100, move forward

a couple of frames, then set it to 0, this will automatically create another

keyframe at a value = 0. Move another couple of frames forward and set 

it back to 100. copy/paste the keyframes for as long as you want the light

to keep on flashing.

you can set the graphic blending mode to add if you are setting it on a video clip.

you can also create the same in after effects and export it as a mogrt (it is easier

and faster to do it in AE as you have the option to loop the keyframes forever)