Plastic Lumber made from 100% HDPE like MAXiTUF would not stain. However, if you are using a composite made with some additional filler such as wood or bamboo you would be able to stain the stain the filler.
Plastic bottles and containers are usually marked with the Resin ID Number on the bottom (1 to 7 ) within a triangle with three chasing arrows. That sign does not mean it can be recycled. It merely...