I have seen flowers in stores that are unnatural colours and would like to know what chemicals are used to do this and can it be done to still growing plants. I seem to recall seeing some chrysanthemums in Japan that I thought were changed this way.
Florists use several ways of changing the colours of cut flowers that include using dyes and sprays. A simple way to change the flower’s colour is to cut the bottom of the flower stem on an angle and place the flower stem in a solution of water with food colouring. Leave it for several hours or overnight under lights and the flower will gradually change colour. What happens is that as water evaporates through the leaf pores (stomata), fresh water is drawn up through the xylem in its stem bringing the dye along with it. This works best on a pale coloured flower. There are videos on line showing how to do this.
Florists often use stronger chemical dyes. For the amateur florist you can find dyes in a craft shop, however be aware that these dyes may not be food grade so should be treated according to the safety advice on the label.
As for changing the flower colours of growing plants, this can be done with hydrangeas, changing the blossoms from pink to blue by adjusting the soil pH from alkaline to acidic. See the link below. A few other flowers also change colour naturally. For example, Hibiscus flower colours change with changes in temperature. As temperatures rise more carotenoids are produced resulting in more vivid yellow, orange and red colours.
I have not been able to find any other flowers whose colour can be controlled like that of the hydrangea with changes in pH. Most research seems to be geared to making genetic changes to produce unnatural colours like blues in chrysanthemums.
https://www.torontomastergardeners.ca/askagardener/hydrangea-and-its-changing-colours/
https://www.hiddenvalleyhibiscus.com/misc/colors.htm