Boab, you can make that red underlining go away if you're working in MS Word; go to the Tools menu, click on Language, and then select your favoured variant of English from the drop-down list, which is very comprehensive!

Many standard variations in English spelling (e.g., colour/color, kidnaped/kidnapped) arise from 19th-century American efforts at language-rationalization that were disseminated and popularized by Mr. Noah Webster and his dictionaries. Similar campaigns were waged with great success in Germany and the Scandinavian countries. There are two big reasons (and many small reasons) why Britain was rationalization-proof: first, during the 19th century the educated classes learned grammar and syntax in Latin and Greek, not English; and, second, the first truly comprehensive and reliable dictionary of British English was the Oxford, which operates on the basis of historical precendent -- that is, it traces the development of words and their meanings from their appearance in written sources. Oxford doesn't DO reform.

Canadians, being bombarded from both sides, have to use both sets of dictionaries and therefore have twice as much opportunity to get it wrong.