The short answer to that question, is no. There are many variants of English - though people are most familiar with the British and American forms.
Contrary to partisan opinion all variants of English are correct, but it is essential to use the same variant consistently throughout the whole of the document.
As a rule of thumb you should use the "brand" of English which your target reader is likely to feel more comfortable reading. You want them to read what you've written without giving them reason to pause at a word which is spelt differently to what they think it should be.
A stricter rule should be applied when writing or editing formal documents for use by an organisation which predominently uses a particular variant of English. For instance, if the document is a report for use by the EU then, as the UK is a member nation of the EU, British English is the form which should be used. If, on the other hand, the document is a report of some description for an organisation based in the United States, then US English should be used.
There are also variant forms of English which contain both British and American spellings, as used as the house style of some corporations. BP, for instance, generally use American English as part of their global branding, but they also retain British spellings for a clearly defined number of words such as 'colour', 'centre', 'litre', and 'metre'.
So if you happen to see a mixture of spellings in some cases it might, actually, be meant to be written that way.
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