What type of wood/stain is this?
As bib stated, it is in the color/grain of the wood.

If you used red cedar, redwood, and perhaps douglas fir the striping will come naturally if the sap wood is included. The darker wood is the heartwood, towards the center of the tree, the lighter is the sapwood, which is around the outside. My guess is that redwood was used. No stain at all. The finish coat will change the color a bit, enhancing the grain of the wood. Most outdoor "clear" finishes give some "ambering" to the wood it covers.