Here's why:
* Historical reasons: The spelling "colour" is based on the Old French word "color," which itself came from the Latin word "color."
* Consistency: British English tends to maintain older spellings, even when they differ from American English. This is seen in other words like "centre," "metre," and "theatre."
Why is it spelled "color" in American English?
* Noah Webster: The American lexicographer Noah Webster, who created the first major American dictionary, favored simpler spellings. He felt that the "u" in "colour" was unnecessary and changed it to "color."
* American Revolution: The American Revolution contributed to the rise of a distinct American identity, including language. Changing spellings like "colour" to "color" was a part of this movement towards independence from British norms.
In summary: The spelling of "colour" vs. "color" is a matter of national spelling convention. Both spellings are correct in their respective English variants.