Food coloring will dissipate to quick. I used a product call tracer dyes to locate pool leaks.
Hammerlane. Note the original thread is four years old but was resurrected, with a modified question yesterday and others through the years. The new poster does not seem to want to chase down a leak. He wants to know if he can just put a patch or seal over a light opening once the fixture is disabled. As I understand it anyhow.
Just to also confirm, I agree that it is the light niche. This is a very common problem pool owners have. As a matter of fact, I had this problem earlier this year. Leak detection company came and fixed the problem.
As for the dirt washing out, it's simply a matter of back filling and packing the ground.
Something to consider:
If you already have to, say, dig the ground for this work, go ahead and construct and install a french drain system around this area. This will help with the future incidences of preventing the ground to wash out again...