Hello,
I have never tried but I have heard that it is a good method to find out if your toilet it is leaking water.
Personally I use another system to find out if there is any water leak at home.
When everyone is at bed and nobody is using water at home, I take note of the count from the water meter. Then, I come back after one or two ours and take note of the count again.
If the count hasn’t changed, all good. If the count has changed that means that there is a water leak somewhere and I need to check the usual suspects.
Instead of buying the coloring to find out if you have a water leak in the toilet, first check the water meter to know if you have any leak at home. You may safe yourself some money…
For more info, have a look to https://www.effiworkx.com/water/