Wikipedia:Spelling
The spelling used in the Wikipedia can be either British English or American English.
It is best that you write in the version you are most used to.
The simplest thing to do is to try not to mix the two. It will look bad if we have some of the spelling and grammar of one country mixed with that of the other country. This is why an article should be written in what you are used to using, because otherwise you might spell some of the words in the British English way, but not realise that other words you are using are American English, or you might know what the British English spelling of a word is but not know that if you are writing in British English, then the grammar should also change.