Water color additive that will maintain color when water is a thin film
Use fluorescein or other fluorescent dye, along with an effective UV filter (AKA haze or skylight filter) on the lens and an ultraviolet source, such as a xenon strobe (speed light) or UV flashlight. A UV-transmission filter on the strobe can increase contrast, and can be made cheaply, but might not be needed. Because fluorescence makes the fluid become a source of light, it is quite obvious against a dark background. You can even use most laundry detergents, which contain blue-fluorescing dyes -- though foaming might be a nuisance. 

See this image of fluorescent soap bubbles or this one. Soap bubble film may be just a few hundred nm thick, yet show clearly with fluorescence. Also watch a demo of strobe illumination of water droplets using sodium fluorescein for contrast. In the video, notice, in particular, that the stream tends to break into pairs of droplets, one perhaps a millimeter in diameter, and the sibling, just a few microns across, but all clearly visible! 

Choose a near-UV light source and fluorescent dye for safety, since shorter wavelengths are more damaging to eyes and skin. A search for UV flashlight returned items from US$10 and up, some of which might be suitable. Most were in the 395 to 365 nm spectrum, relatively innocuous.