What does metabolic sand do to a savannah monitors? i hear they change the color of the sand when they eat it?

my friends uncle told me that he had a savannah monitor and he had red and black metabolic sand and when the monitor would eat, the sand would be ingested to and the monitor would turn the color of the sand eaten. in this case, red and black. i want to know were to find this sand for purchase and if it even does what they say it does.