Add a red flash when tracker moves off of a path

[amir]

We have people walk on a balance beam with an Optotrak rigid body which tracks their movement. We want the display to flash red whenever they step off to the side a certain amount (x-direction). The flash would look like what is available in The Pit script. How can I add this effect for when people step to the side and the rigid body deviates off the path?

I'd also like to be able to toggle this function off and on as the script is running to remove any potential for flashes when need be.

Thanks!