Grey is English, Gray is American (thing)
I noticed as a young reader that some books spelled words differently to others. I soon realised that American authors often chose to spell grey specifically with an ‘a’ instead of an ‘e’ whereas (it seemed) British colonization chose to spell it “correctly”. I have, since that awareness, been sure and then unsure of its spelling until I did some research.
I found that there isn’t in fact a “correct” spelling of grey. Not even in the Old English was it distinctly spelled grey or gray but rather grǽg. Adding to that, both spellings were in use for hundreds of years until recently in the 19th century when the distinction came about.
Most words which include the achromatic colour (such as greybeard/graybeard) may be spelled with both the ‘e’ or the ‘a’ with the exception of the greyhound dog breed, which always has the ‘e’, the spelling of a proper noun (e.g. Earl Grey) and also in food irradiation where Gray is used to measure the quantity of radiation energy.
At the grey or gray website there is a historical footnote about “gray” having been the chosen spelling before the 20thcentury by English Lexicographers and especially by the English writer Samuel Johnson. It also mentioned that “gray” was standardised earlier still in the US using an examination of two 19th century dictionaries* printed there, that suggest the reader look up “gray” at the entry for “grey”.
The grammarist has some interesting charts on the use of grey/gray throughout the centuries. Check 'em out if you are a fact fiend.
thanks for the feedback wertperch!