Hi,
I have never heard of anyone trying this but as always you should be cautious of introducing any new chemicals into the process as one never knows how they will react to the other chemicals such as the sulfuric acid and such.
If this a Black and White booth, most likely any coloring you put in any tank will be neutralized by the bleach (#6 tank) or toner (#9 tank). You could try a high amount of food coloring in the #10, #11, and #12 tanks as they are the final rinse and are just plain water.
In a Color booth, you could try putting thin, transparent pieces of plastic in front of the camera lens, using them as a color filter.
Good luck in your artistic endeavor!