Question: 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?

Answer: 
Sure. For example, [Canadian English](http://en.wikipedia.org/wiki/Canadian_English#Spelling) has standard spellings that are derived from both British and American influences.

> 
> Canadian spelling of the English language combines British and American conventions.
> 
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> * 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](http://www.canadiantire.ca/en.html), 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*](http://search-recherche.tc.gc.ca/search.aspx?q=aeroplane&cn-search-submit=Search), while the CBC (media) and everyone else in Canada use [*airplane*](http://www.cbc.ca/gsa/?q=airplane).
* It looks weird to someone who is used to checking spelling, for example anyone who produces or checks professional copy. They're used to noticing single errors like typos, and common mis-uses like "flout" vs "flaunt" in everything they read, but they might well perceive what you're doing as a consistent mis-spelling.
* Individual words might look weird (but whatever spelling you use, *someone* who's used to the other will find it weird, so this may not be a problem).
* It's "wrong" in the sense that it's not correct British English and it's also not correct American English. But from what 200\_success says, almost any mixture is correct Canadian English, so you can plead that. Furthermore, only people very familiar with multiple versions of international English will be absolutely certain that what you're writing isn't conventional *anywhere*. So you can get away with being slightly "wrong".

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.