How to create different fluorescent colors using household items
If you dismantle highlighter pens, you can find the felt material inside. This material contains fluorescent chemicals, such as pyranine. This felt like material is usually encased in a plastic film, remove the film and soak the felt in water.

If you use too much water then it may become too dilute and you may start to encounter an issue with the ink/water mixture sticking to the flower. 


Are you using a black light to view the fluorescent colour? 


it might be worth trying to cut the stems and keep the flowers in the ink containing water. Then as they absorb the water they will absorb the fluorescent ink as well and may take on some of the colour (and viewable under black light).