I would think the easiest way to get color into them would be as soon as you cut them, put the cut stems into vases full of colored water and it will go up the stems. I used to do that with carnations, and you could even split the stem into sections, put each section in a different color and get a flower with different sections of color in the petals. The coloring was never really noticeable on green parts of the plant though, only in the white petals.

Putting it in the soil I would suspect may not work if the roots don't absorb it through the cell walls. With cut stems it's just an actual series of tubes running up the stem, and they are open-ended instead of having to go through a membrane.

Is food coloring safe for inhalation/smoking though?