English v American
I think Tinycards is useful but find it annoying when it doesn't accept things like e.g. Sweet - it wants Candy. Grey it wants Gray. I know it is an american course but lots of english people participate.
It's not English versus American, it's British English versus American English.
I haven't used Tinycards very much, but on Duolingo, British variants are accepted, and if they're not you should report them so they can be added.
That said, Duolingo is an American company and the user base for American English is much bigger, so we are not their highest priority. Learning to use American variants is not that big a deal.
As I say, I can't speak for Tinycards - I don't know what the situation is over there - but here on Duolingo, if a British spelling/alternate isn't accepted, then if you report it, it is generally added. I haven't used Tinycards very much at all, but I imagine the same policy would apply there if there is the facility to add alternates. If they don't have that facility, then you're just going to have to grin and bear it.
I prefer it when British English alternates are accepted myself, but given the wide varieties of English worldwide, where does one stop? Duolingo is an American company, and it's best to assume that American standard English is what is expectedly. It can be frustrating, but we can't expect them to cater to Britons "just because."