Well, both are fine, but in the UK, we tend to prefer grey while across the Atlantic they tend to go for gray. Apparently, some people use both spellings to distinguish different shades of the colour, which seems to be overcomplicating the whole thing. You can see more about it here.
I’m not sure why the success of Fifty Shades of Grey can be said to increase the uncertainty. Doesn’t that merely reinforce the validity of the e option?
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