The quality of the British English spellchecker seems to have deteriorated in the MacOS Mojave beta. Certain American spellings, especially the notorious duo of color and center, aren’t being underlined as errors. In previous MacOS releases, they were. I’ve noticed a similar problem in iOS 11, though it’s limited only to center. This has been a problem for a while – canceled has never been marked as an error, for example – but at the very least the spellchecker was able to recognise that archetypical American spellings should be underlined.
If you find the inconsistency as annoying as I do, you’ll want to download an alternative dictionary instead. This StackExchange thread links to LibreOffice’s British English dictionary and provides instructions for installing it. While the LibreOffice dictionary is somewhat smaller than the pre-installed Mac dictionary, it is more accurate at recognising US spellings and marking them as errors. Another perk of the LibreOffice dictionary is that it allows spellings like realize and organize through. While -ize endings are sometimes associated with North American spelling standards, they’re actually the preferred variant of Oxford University Press, and Oxford dictionaries will always list the -ize spelling first. The default Apple dictionary underlines all spellings that end in -ize.
I filed a bug report with Apple, since I think they may have made some mistakes with the dictionary file. As far as I can tell, Apple’s English dictionaries are based on a US English master file, and the British, Australian, Canadian and other localised dictionaries exclude the American spellings and add Commonwealth ones like colour and fibre.
If you use one of the other non-US English dictionaries on the Mojave beta (or even High Sierra and earlier MacOS versions) and see obvious American spellings being let through without underlining, please let me know so I can update this entry. Tell Apple, too, so that they know to look for the issue in forthcoming releases.