You can buy water dye in liquid or tablet form. Used in public works application. I actually have some tablets from my past tracing underground storm drain tie-ins to outfalls. Probably the same stuff they use in holding tank overboard detection.
Ask a public works guy. Got mine from Acklands-Grainger.
For low volume, could use food coloring at a high dose.
Can also buy a cheap UV flashlight which might help with some color detection.