It really depends on the tartan, for example with the Isle of Skye I wear Purple hose, I really do not see me wearing them with any other tartan that I own, and with my Maple Leaf I wear Maroon hose that would not work with any other tartan, but generally speaking
Black works well with almost anything, Bottle green, Lovat blue and Lovat green are good choices as well for many tartans, if you want to get daring you could always go for Red hose though they are not for the faint of heart. As for flashes, I would say check out the traditional garter ties at the Scottish Tartan Museum, they are very comfortable and come in a lot of colours that lend themselves to many different tartans.