Kodak C-41 first rinse; color of rinse water

The light sensitive goodies in all emulsion layers are various salts of silver. In their natural state they are only sensitive to violet and blue light. These have been modified to increase their sensitivity into the green and red regions of the spectrum. This is done by inducing sensitizing dyes into the mix. These dyes plus a back coat (antihalation coat), made with dye is designed to protect the film from secondary exposure from the rear. Halations are unwanted reflections caused by light traversing the emulsions and then hitting the pressure plate and/or junction emulsion to film base and reflecting backwards into the film. A halo-like circle will surround bright areas. This layer stops this action.

All these dyes are water soluble and like all the other chemicals and residual stuff are reasonably benign.