Plants change color according to their pigments. You are assuning that a) the food coloring is a molecule that the plant can actually uptake, b) that it will not metabolize it into something colorless c) that you can add enough (assuming a and b are true) to see a actual change and d) that the food dye is safe to smoke, which is different from being safe to eat.

Grab a bottle of SnowStorm (seaweed derived plant hormone), foliar feed your plants with it, drop your temps down to 60 at night, and watch the magic. This only works on plants that are genetically capable of producing the dark pigments.