WagonTrainToys.com has wagons made with interesting materials including plastic and fiberglass (see here) with color options that include pure white. These wagons resist the effects of sun and rain exposure that can promote oxidation or decay. Hochstetler's interesting and environmentally cool variation in wagons uses "boards" of weatherproof recycled plastic.   

The plastic racks come in a variety of colors: Red, blue, dark green (shown, right), green/yellow alternating, black/gold alternating (shown below), and black/red alternating.  Notice that the black combinations have more black in the bed. 

Colors are not as saturated as paint but don't seem to fade and are through the material. Novel pinstriping is of white pavement marking paint with crystal grains, making the trim reflective. Call for availability and shipping time on custom white racks for a fully white wagon. These are especially nice for weddings to pull the flower girls, though some brides still choose colored racks if they match the wedding colors.  

On a more everyday note, plastic is ideal for livestock feed or water hauling because it will not rot and "Pony Kiaz" won't be as tempted to chew on the wagon!