There is nothing that infuriates the players of word games (https://wordleunlimited.dev/)
more than entering a word they have been using their whole lives, and receiving a message that it is not a word. And that is what happens to British, Irish, Australian, Indian and Canadian players in most word games – and it is not a bug, but a problem yet to be solved.
The point where the two varieties diverge
The differences are consistent and predictable, which means you can learn them and stop spending a guess on this issue.
The last row is what causes most problems in five-letter word games. Both GREY and GRAY are acceptable, common, five letters long and potential solutions. If you submit GREY and receive grey tiles, you are learning a wrong lesson about the word.
Why word games handle it differently
Almost all word games use some of the public word lists as the basis for their operation, and those lists have a preference for a certain regional variety programmed in them.
Some of the games accept both versions, but use only one of them as the answer. This is the most common solution, and also the most sensible, even though it means occasional wasted guess in learning about the variety of the answer list.
Some games accept only one variety, and this is just annoying.
Fewer still allow you to choose your variety, and this is the right solution, and much less common than it should be.
The effect of the difference on your game
If you do not know what variety of the game you are playing, there are several ways to protect yourself from problems.
First of all, do not use ambiguous words as openers. GREY is a bad opener not because of its letters, but because of its ambiguity. Pick something else.
Second, be suspicious of OUR and RE endings. If you are playing the American variety, you will never see COLOUR and CENTRE among the possible answers, so do not waste your guesses trying to use structures that won't be there.
Third, remember that both varieties are acceptable as guesses, even if only one variety is used as the answer. You can test your letters using British spelling; it is just a word you will never submit.
Fourth, pay attention to Z. If you use ISE endings as your answers, Z will not be nearly as valuable in British English as in American word frequencies.
Why the regional variety is so important
The ability to choose your variety is not a matter of taste. It will affect your answer list and therefore your strategy.
Wordle Unlimited has a regional variety and language settings in the settings menu, under the gear icon. This setting is especially important for people from countries outside the USA, who have to spend part of each game submitting their spelling and getting the message it is incorrect.
But this setting is even more important for teachers. Class learning English according to the British curriculum, but playing an American word game will find their students submitting their spelling and being frustrated because of it being marked as incorrect.
The wider issue of word lists
All word games use two word lists. First, the long list of words which can be used as your guesses. Second, the small list of words that can be used as answers.
Guess list has to be sufficiently large to include the obscure words, because they can be used as guesses. Answer list has to be limited to words known by most people, because a puzzle with an obscure word as an answer is not very exciting.
That is why sometimes you can submit an obscure word and have it confirmed, but never see it as an answer. And that is why most disputes about correct words end up here. The game will confirm the existence of the word, but it will never use it as an answer.
What is reasonable to expect
No word game can satisfy everybody on this question, because English language has several standard varieties, and the puzzle has to choose between them.
What is reasonable to expect is acceptance of both spellings as guesses, indication or configurability of the choice of the variety of the answer and lack of flagging one of the varieties as an error. Wordle Unlimited solves this issue by providing a regional variety setting in the gear menu at wordleunlimited.dev, along with word length, theme and hard mode.
Apart from that, it is something you need to know rather than argue about. Knowing that GREY and GRAY are both valid options is just playing a word game in a language with two spelling varieties.