Food colouring has been used for years to change the colours of some flowers that are sold at grocery or variety stores. The dye's affect on plants, through a process called transpiration, has been the topic of many children's science experiments for just as long. Food colouring is more likely to affect cut flowers than potted ones, which is why some stores would rather inject the dye straight into the flower rather than colour the water in which the flower rests.
Transpiration occurs when water is pulled up through the roots and into the leaves and flowers of plants. According to the Steven Spangler Science website, the dye then moves up the stem---over a 24-hour period---and throughout the plant, causing a colour change in the areas of the plant that have absorbed the water.
Effects of Different Colored Food Dye
Blue food colouring works best for the transpiration of dye into a plant's blooms. According to a study done at Bishop Dwenger High School in 2007, potted plants that were given blue-dyed water, tended to show more blue in their stems and leaves, than those given red or black dye. Another interesting finding was that although blue and black dye had no affect on the growth of the plant, red dye slowed the plant's growth.
What Type of Flower Works Best for Food Coloring
White carnations tend to be the best flower to use if you want to the colour to be deep and to affect the plant quickly, though any white flower will show the most intense colour changes.
Other Plants that Work
Celery is the best vegetable to use when experimenting on colour changes through dyed water. The vines on the stalks of celery are very pronounced, making the dye that much easier to see.
An experiment done by four high school students in a botany class yielded interesting results when using red dye on sunflower seeds. When the seeds grew into plants, their stems were tinted pink.
Injecting Food Coloring
Some florists use cream-coloured roses and inject the buds with a variety of food colouring shades. When the flower opens, the rose takes on a tie-dye effect. The dye, however, does not affect the roses stem or leaves.