light grey or sand or light brown... and like keakar says a flat color... anything to cut down on that deck getting so hot it will blister your feet.
Here's what i did, turned out pretty well and other than one problem spot (that i either didnt sand good enough, or where i didnt wipe it down right) has held up great.
alternatively that stuff they sell at home depot / lowes (the new textured deck coating stuff) is supposed to be real good as well.