Many external factors can influence colour: light, temperature, pH, sugars and metals. There is a method to turn petunia flowers from white to transparent. The petunia flower is immersed into a flask of water, connected to a vacuum pump, after which the flower appeared colourless. The white colour is expressed by the air present in the vacuoles that absorb the light, without air the flower loses the white colour.
There is an increasing interest in flower colour, since some colorations are currently unavailable in plants. Ornamental companies create new flower colour by classical and mutation breeding and biotechnological approaches. For example, white bracts in Poinsettia are obtained by high frequency irradiation.