American usage of the English language (US English) differs from British usage (UK English). In addition to differences in vocabulary, there are differences in the spellings of words. For example, Americans use the word color, whereas the British use colour. Similarly, visualize occurs in US English whereas visualise occurs in UK English.
Whilst you can process these differences subconsciously and not even notice them when reading a book, Microsoft Word will underline in red what it considers a spelling mistake.
This can quickly become annoying, and distracting. When trying to locate and correct errors in your document, Word will erroneously highlight correctly spelled words. Many people simply put up with the annoyance and try to ignore it. But you don’t have to.
You can tell Word that your proofing language is different.
If Word highlights the word colour as an error, and you are British, you should set your proofing language to be UK English. Do this by going to the Review tab and in the Language group click Language > Set Proofing Language.
In the language window that opens, select your preferred proofing language. If you click ok, the change will affect only the current document. However, you should probably consider setting it as default: click Set As Default. Doing this will set the default proofing language against the NORMAL template and so will affect all new documents. This is probably what you want.
If you change your mind, for example if you are working on a document for an American audience, you can always change the proofing language again using the steps shown above.