Good idea ..
Fresh beet juice would be most likely the best choice... mixed with oil/grapefruit juice ... it is natural , and is far from green/yellow/blue/black color that most stones are composed of.
Even carrot juice can color stool, but is not as strong.
Raspberry juice can also be an option.
Or, red pepper powder.
I would try it, but my last 2 flushes have not produced almost any stones ... so I guess I will have to wait for a 3 - 6 months, to test it.