I saw an experiment here on blip. It was about colouring white flowers with the help of coloured water. I learned, that you can use solvent-free ink. The colour particles are fine enough to get through the water "pipes" in the flower and colour them. You can see the experiment in the extra of yesterday blip.
I really like the result. Funny enough only this tulip got really colourful. The black and the blue ink didn't work, there were only very tiny spots on the leaves.