Is it "gray" or "grey" hair?

Whether you write gray hair or grey hair depends on which dialect of English you are using. In American English, the standard spelling is gray hair, following American spelling conventions that use gray with an a. In British English, Canadian English, Australian English, and other Commonwealth varieties, the standard spelling is grey hair with an e. Both spellings refer to the same hair color and are correct within their respective dialects, so neither is wrong. If you are writing for an American audience, such as in the United States, use gray hair to match American style guides. If you are writing for a British, Canadian, or Australian audience, use grey hair to match British style. Many people encounter both spellings due to international media and products, but for formal writing, you should be consistent with your chosen dialect throughout the document. So you can write either, but choose based on whether you follow American or British English conventions.