I bought some white roses, daisies and chrysanthemums from my local florist (they have the best cactus!) and used a generous amount of food dye (neon, fall, and standard) to color the water. These dyes were given to me when a friend was moving out of the country over 6 years ago and I've never used them until now.
The flowers almost immediately started to change colors. You can see the slightest hint of color in the picture above. The purple neon dye (second to the right) didn't work. After a few hours most of the flowers were already brightly colored, but not the purple. Even after I switched the water and the flowers didn't want to absorb dye. So avoid purples. The roses seemed to be the most receptive to the dye.
The picture above is after only an hour of dyeing, and below is how they looked after dyeing overnight. So bright & colorful!!
This is so easy and fun & would be a great project to do with kids, too. Let me know if you give this a try!