I normally write using the American English forms, but for some particular words, I tend to naturally write it in the British manner (like with the word favourite rather than favorite).
Is it wrong to mix both styles? If not *wrong*, does it looks weird?
Sure. For example, Canadian English has standard spellings that are derived from both British and American influences.
Canadian spelling of the English language combines British and American conventions.
- French-derived words retain British spellings (colour or centre). While the United States uses the Anglo-French spelling defense or offense (noun), Canadians use the British spellings defence and offence (defensive and offensive are universal).
- Use of curb and tire in contrast to British English kerb and tyre.
- Words ending in -ise use "-ize" after the US English convention.
- Some nouns take -ice while matching verbs take -ise, as with practice and practise, licence and license.
- Canadian spelling can retain the British practice of doubling consonants when adding suffixes to words even when the final syllable (before the suffix) is not stressed, as with travelled, counselling, and controllable.)
The preferred Canadian spelling is favourite. That is the form that would be chosen in formal publications. However, in practice, Canadians tolerate both British and American spellings with little fuss, and favorite would also be commonly seen.
Note, however, that there are a few words for which Canadians do have a definite spelling preference. Kerb and tyre would look markedly out of place in Canada. I speculate that it's because they are more modern concepts, and therefore subjected more to American influence. Also, there is a popular chain of hardware stores called Canadian Tire, which would make tyre un-Canadian.
Caution: "Official" usage doesn't always paint an accurate picture of common Canadian English usage. For example, Transport Canada (government) regulations all use aeroplane, while the CBC (media) and everyone else in Canada use airplane.
I think most people would recommend you stick to one style or the other. Why? Well, it doesn't matter too much, but if you mix styles the reader might notice! And that's bad—if they're noticing stuff like that, then they're paying attention to how you're writing rather than what you're writing.
In other words, you're distracting the reader. You probably don't want that. You want them to focus on what you have to say, right? So unless you have a specific reason to mix styles, just stick to whichever style seems appropriate.
I would say that in your own informal writing you can do as you please. Few readers will both notice and care, so even if it looks a little odd it won't seem definitively incorrect.
If you're writing formal English then you should pick a spelling convention and stick to it. It's part of a style guide. This goes beyond even choosing British vs. American: for example there are plenty of words which in Britain can correctly be spelled either -ize or -ise, but generally you should spell all of them the same in a given text, and certainly you should spell each one consistently. This is something that spelling/grammar checkers in my experience don't pick up on: "you've used 'spelled' in this paragraph and 'spelt' in that one, would you like to be consistent?".
So in practice, many native speakers make mistakes along the same lines, especially a Brit trying to write American English for an international audience. Therefore, although it's better to stick to a common set of spellings, it won't stand out all that badly if you don't.
It must be noted, to add to the other answers, that some official exams will penalise you for mixing. For example, Cambridge (British English) will probably tolerate American English if you are consistent, but certainly penalise you if you mix them. I have also known of professors in the UK that consider American-spelled words an orthographic mistake, and therefore lower your grade for it (and being harsh!).
I know a German girl here in Florida, who obviously learned British English in school. But after living in Florida for over 10 years, she has an immaculate American accent. The problem is that she still uses "bloke" where Americans would use "guy," she says "you know 't I mean?" with a British intonation but American accent, she says "zed" instead of "zee" and says "shedule" instead of "skedule." You would think these innocuous little details that nobody really notices, but the fact is that a lot of people find her amusing and even laugh at her (even some of her friends.) I personally find the inconsistencies jarring - especially "bloke." Every time she says it, I feel like saying "don't say that!" hahaha
I would suggest that there shouldn't be any differences in the patterns you use as patterns are an important part of understanding what people are saying. For example, don't say "my favourite color", or "my favourite colour is red and my favorite shape is a circle".
As a person who is very international, I have nothing against using words used in different areas(garbage/rubbish, etc.), and can accept that it may seem weird to use say Australian and American words mixed within British English, but only see it as an adaption and a sign of the diversity of the person's language and experience, and not something that is wrong.
Language is about 2 things, expressing yourself and communicating. If something that sounds awkward does one of the above better than the standard, then I can not see what is wrong with it. Language adapts, you just have to look at English 50, 100 and 150 years ago.
