Color, and the precise shade and the extent of coloration, largely determine price, along of course with weight and existence of defects. But there are ways of changing the color from not too valuable to more valuable, so you also need to know the extent if any to which the jade has been treated. If you don't know what you're doing -- and even sometimes if you -- there's huge trust involved.
Note that much and probably most nephrite sold in China these days comes not from traditional sources in Xinjiang but from overseas, primarlly Canada. The raw stone is importd then worked to Chinese style domestically.