Because ~TiddlyWiki is of British origin, its English documentation uses [[British spelling in preference to US spelling|http://en.wikipedia.org/wiki/American_and_British_English_spelling_differences]].
Words like <<.word customise>> are spelled <<.word -ise>>, not <<.word -ize>>. Words like <<.word colour>> will also be spelled using UK English unless they are being used for reserved words in code, such as in CSS or JavaScript.