How to grow a Rainbow Rose, Naturally
In 2004, two dutch companies, River Flowers and F.J. Zandbergen, experimented and successfully grew a rose that had its petals rainbow colored. As petals get their nourishment through stem, the idea is to split the stem into several channels and dip each one in a different colored water. This way all the colors will be drawn by the stem into petals and resultant rose will have all the colors in it. The same method can be applied to other flowers especially to Chrysanthemum and Hydrangea. You can use the same idea to color any flower, anyway you like.
Sources: 1, 2, 3
Watch: Flowers Color Time Lapse