Both spellings are correct when referring to the neutral or achromatic color (a color without color somewhere between black and white). Actually, the only difference is the two spellings is geographic preference. People in the US tend to spell the word gray while those humans living in other English-speaking countries tend to spell it grey. I think age plays a factor, too. Older people spell it grey...the way they were taught in school, while the younger generations are learning it as gray.
In the US, the gray spelling appears 20 times more often than the other spelling. The percentages are nearly reversed in the United Kingdom where grey is the preferred spelling. Just in case you need a way to remember this, think A for America and E for England (et al). As in the case with many English words, there are exceptions to the rule.
Exception #1 - Surnames. Gray is always spelled with an "e" when referring to someone's name. For example, you can't spell the tea, Earl Grey as Earl Gray.
Exception #2 - The dog breed. It should always be spelled Greyhound, not Grayhound. The same is true for the bus company bearing the same name. And all specific breeds of dogs should be capitalized.
Exception #3 - As a measure of energy. This last exception applies more to physicists than the average person (or frog). It is the scientific measure of energy called "the gray." "One gray is equal to about one joule of energy radiated by the ionization of one kilogram of matter. The gray replaced the “rad” as a standard measuring unit of radiation energy in 1975. One gray is equal to 100 rads." Got that? Me, either.
When referring to the color, or lack of color, known as grey (gray), there really isn't a rule. But do keep in mind that if you happen to be in England and the Queen is wearing a grey dress, and you write it out as gray, you'll either be considered a simpleton who can't spell...or a rebel!
Whatever you do this weekend under the grey November sky, just be sure to have a little fun and please stay safe. I invite everyone back here again on Monday for another week of what I hope will interesting and informational blogs. But until then, I wish you