The joke goes:
"What's the difference between gray and grey?"
"One is a color, the other is a colour"
It's true when speaking of the color that one spelling is US and the other non-US, but there are other uses that derive from someone's name, and then the spelling is fixed:
- Earl Grey tea is always spelled with an e
- The radiation dose unit gray is always spelled with an a
If you go deep enough, some trades consider grey and gray two different shades!