The spelling used is sometimes called the Queen's English. This form of English spelling is usually the original form of the English word and is still widely used throughout the world in English speaking countries such as Canada, Australia, New Zealand, Hong Kong, India ..etc. to name a few (all former British Colonies). The US/American dialect form of spelling could be considered an exception, in the English language, rather than a rule.

The following are few examples of different spellings

Theatre
Labour
Favour
Flavour
Colour

Cheers