You CAN paint a composite deck.
The purpose of painting a deck is not only for aesthetics but to protect the wood from the elements. The ability to withstand far greater stress from the elements is one of the many benefits of composite deck materials. Another valuable benefit is that composite deck materials do not need to be painted in order to protect them.
Benefits of a Composite Deck
Most composite decking materials come with a natural color built in, so painting to protect them is not necessary. If you simply want a color change, you can paint most composite decking with paint that has been approved for walking surfaces. Any paint or stain that has been approved for wood decks will work just as well on a composite deck. Painting or staining the deck will mean that you won't be able to avoid maintenance and upkeep, which for some homeowners was the reason for purchasing composite decking in the first place.
Don't Rush to Paint
If your composite deck is newly installed, you must wait to paint it until the new composite decking has faded, about twelve weeks, to ensure adhesion.