Window boxes are designed to be a part of the house and as such, color selection is crucial to matching or complementing your home.  Because of this it is often recommended that window boxes be painted in one of four ways:

1. To match the trim around the window
2. To match the color of the exterior shutters around the window
3. The color of the cedar shingles or siding (if you have any)
4. A complimentary color if shutters are not already on the window such as the color of your front door

PVC window boxes are great for holding paint and the paint will not fade or peel.  They're an upgrade over wood which has maintenance issues and vinyl which cannot be painted and have a shiny appearance to them.  It's a good idea to choose a window box color that matches your architectural style and personality as well.  Bright colors will attract people's attention to the window box, but may also make window box stand out more than the flower or plants in them.  Neutral colors allow the window box to function as an accent to the home without being the main attraction.  They can also showcase plants and flowers better.  For PVC window boxes, if you decide to paint them, it is recommended that you use an exterior latex paint.  Semi-gloss is better for minimizing sun absorption and Sherwin Williams even makes a vinyl safe paint that works well with PVC to do more of the same.  Just remember to leave the inside of any window box unpainted no matter what material it's made from.  You never want paint to be in direct contact with soil and water.