A quick google shows that gray is more common nowadays. 

I had the impression that grey was the British spelling ...
maybe I'm just thinking of Earl Grey Tea, though. 

If it matters, gray is more likely to be used in the internals
of programming languages, in color-picker routines and such.

(Edit: Now I'm salivating for some Earl Grey, but don't have
any in the house!  Phooey.  Or phooay, as the case mey be.)

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