By definition, there is no difference between gray and grey, but I’ve always found it confusing when faced with the choice of which variant to insert into my writing, a problem only made more difficult as an American expat in the UK. My language is a muddy mess from both shores of the Atlantic, but, for as long as I can remember, gray has consistently caused me to pause.
In looking it up [insert: once and for all! for dramatic effect] for a current project, the solution flashed into my mind:
gray: the A in grAy is for America
grey: the E in grEy is for England
Perhaps this tip will help you as well. Also, I have a snappy little tip for the difference between i.e. and e.g., if you’d like to have a look.