It's very hard to tell. In fact, Dan Smith has proved this here on italki on a couple of occasions. He posted four news reports on the same topic (US, GB, Can, Aus). There were no obvious cultural references, no spelling clues, and because the language was neutral/semi-formal there were no regional colloquialisms. And you know what? We, as native speakers, couldn't really tell which was which. We were left to pick up on clues such as whether or not metric measurements were used ( which is obviously a cultural rather than linguistic issue, anyway). So, the answer is no : apart from the few minor spelling variations and capitalisation/punctuation conventions, there aren't really any obvious differences in the written language. Informal and conversational written language may include regional slang and colloquialisms, but a newspaper article, business letter or academic text won't. The standard grammar of English is the same wherever the speaker comes from. In my job, I spend a lot of time looking closely at written texts originating in different parts of the English-speaking world. If there are no obvious spelling clues, I can often read an entire article on science or history without knowing whether the writer is British or American. And when I do find a clue, it's often something quite small, such as whether "likely" is used as an adjective or adverb, whether a collective noun is treated as singular or plural, or the odd irregular verb such as fit/fitted. These are very minor differences which many people wouldn't even be aware of.

Through written text, it is much harder to tell where a person it from. While there are are some vocabulary differences between British English and American English, those differences can't necessarily be relied on to tell where a person is from. Since people from places like Great Britain, Australia, New Zealand, and (in some ways) Canada share commonalities in their vernacular, we can only make guesses when talking to someone through a text-based conversation. Even without certain words or expressions, there are always spelling differences in some words that are considered standard. Such as the example you gave: color/colour In the USA, people spell it "color." This is considered standard and correct in America. In Great Britain, Australia, and most other English speaking countries, people spell it "colour." This is considered standard and correct in those countries. No matter if you get rid of particular vocabulary or vernacular usage, these differences will always exist between countries. Grammar is pretty standard no matter which English speaking country you go to as well, so relying on grammar alone, it would be impossible to tell where a person is from based on that.

My question is this, "Why would you want to know the point of origin of the author of the text?" Would it not be enough if the writing was coherent and grammatically correct?