gray and grey - the difference
The Quick AnswerIf you're following US writing conventions, write gray.
If you're following UK writing conventions, write grey.
Gray or GreyThe colour ("color" if you're an American) between black and white can be written gray or grey.
In the US, the spelling gray is far more popular than grey, and this has been the case since at least the early 19th century. As a result, many Americans consider grey to be a spelling mistake.
Outside the US, the spelling grey (which has been around for nearly 1,300 years) still dominates.
Top TipIt's Not a Serious Mistake to Use the Wrong Version of Gray or Grey
There is notable leniency with the spelling of grey and gray. Grey is an accepted variant in the US, and gray is becoming increasingly popular outside the US.
However, for now, to keep safe and to ensure you don't annoy your readers, use gray if you follow US writing conventions, and use grey if you follow UK writing conventions.