To dye any flower, you add food coloring to the water they are sitting in. However, daisies have a yellow center, so when you dye the petals blue, the center turns green. This is how florists get blue carnations.
Not really sure how to...but I remember putting johnquils in ink as a school project ..so maybe put them in water with food dye?
I know the florests spray a lot of white flowers different colors...probally using a food dye in a little airbrush.