Grey is an alternate spelling for the word gray, which is a color derived from mixing white and black. The word has also come to stand in for other mixtures as well. A person in a grey mood feels gloomy.
If you are commanding a military force and you’re not sure whether you should attack a crowd that is half enemy fighters and half innocent citizens, you could say the decision falls in a grey zone. Grey can be a verb too. When you get old, your hair will grey if you don’t go bald first.