Try spray paint. They make spray paint for plastic now.
Hi this is Peggy.. Hope this information helps you paint your plastic flowers. Hold the spray paint 6 to 8 inches above the flowers and spray the paint evenly amongst the petals. Let the flowers dry, add acrylic gloss spray to individual flowers, saturate the petals with a double coat, and let dry. Replace the dried flowers onto the original stems.
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Rusteloum makes a good spray paint for this and so does Krylon. If you put them inside a cardboard box you don't have to worry about any over spray. Several light coats are better then one or two heavy ones since you don't want any runs. Best to this outside for good ventilation.
Spray paint that is labeled for plastic.
You could spray paint them