This color defined in Overlay Results (Blob).vi which is called from IMAQ Count Object - this is color box named "Center Color" and red is "hard coded".
You can just change this color and make this new color value as default, the save SubVI. But its not a good idea to change VI in vi.lib. Better way - make your own copy of IMAQ Count Object and Overlay Results and change these SubVIs as you want.
Andrey.