Material nodes:
World time or game seconds or something like that.
Hook it up to Sine function. This will spin a value over time between -1 and 1 forever.
Plug that value into a multiply node. Plug your yellow color node into that multiply node also.
Plug the multiply node into an add node. Plug the character skin texture into the add node also.
Plug the add node into the Base Color of the Material node. (I hope this works)
Now it will always flash, so you need a way to activate or deactivate the flashing.
To do that, create a scalar parameter node and plug it into the multiply node also. Then in blueprint you can turn it on and off by setting the scalar parameter to 1 or 0.
If you have more than one character that can use Star power and they need to flash individually instead of all of them flashing when one of them has star power, then your character blueprint will need to create a Material Dynamic Instance on their Beginplay, and change the scalar param on that MDI (make sure to promote it to a variable where you create it) instead of on the original material.