Looking for a non-toxic, environmentally safe highly colored or intense dye
Many organic dyes which are fluorescent can be carcinogenic like the famous Rhodamine dyes. You can try a couple of tricks. One of best inks, at least for demonstrations, is the yellow highlighter's ink which fluoresces bright green in water (see my photo below). If you can shine a bright white LED or a bright blue LED, it will fluoresce very strongly. You can use an optical filter in front of the camera that can block blue light and let the green and longer wavelengths in. With that you can increase the sensitivity.
Note that fluorescence intensity is directly proportional to the incident light intensity in dilute solutions.