Anyone know of a water-soluble dye that either changes color when in water (vs dry powder form) or fluoresces?

I have already tried fluorescent water tracers, but these do not work because the dry form fluoresces already, so I cannot take a picture that will separate the water from the dye particles by color. What I am hoping for is some sort of inert dye that will carry out some reaction in water to allow colors to appear. I will be taking pictures with a camera, so I don't want the dye particles to be visible unless it is dye-ing the water.