The colours aren't important. The change is. It's auspicious because Yoda is otherwise shown with brown robes in the other episodes. But if you like, the grey cloak puts me in mind of shrouds and rainclouds. Brown is more like copper or sulphur: of the Earth. You can -- and should -- draw from that the inferences you want. Like a restless wind inside a letterbox: pretty much the visuals of Star Wars (morphing shapes and colours in a 2.35:1 frame).